diff --git a/dvc/command/metrics.py b/dvc/command/metrics.py
--- a/dvc/command/metrics.py
+++ b/dvc/command/metrics.py
@@ -3,7 +3,7 @@
 import argparse
 import logging
 
-from dvc.exceptions import DvcException
+from dvc.exceptions import DvcException, BadMetricError
 from dvc.command.base import CmdBase, fix_subparsers, append_doc_link
 
 
@@ -16,6 +16,10 @@ def show_metrics(metrics, all_branches=False, all_tags=False):
         metrics (list): Where each element is either a `list`
             if an xpath was specified, otherwise a `str`
     """
+    # When `metrics` contains a `None` key, it means that some files
+    # specified as `targets` in `repo.metrics.show` didn't contain any metrics.
+    missing = metrics.pop(None, None)
+
     for branch, val in metrics.items():
         if all_branches or all_tags:
             logger.info("{branch}:".format(branch=branch))
@@ -32,16 +36,17 @@ def show_metrics(metrics, all_branches=False, all_tags=False):
             else:
                 logger.info("\t{}: {}".format(fname, metric))
 
+    if missing:
+        raise BadMetricError(missing)
+
 
 class CmdMetricsShow(CmdBase):
     def run(self):
-        typ = self.args.type
-        xpath = self.args.xpath
         try:
             metrics = self.repo.metrics.show(
-                self.args.path,
-                typ=typ,
-                xpath=xpath,
+                self.args.targets,
+                typ=self.args.type,
+                xpath=self.args.xpath,
                 all_branches=self.args.all_branches,
                 all_tags=self.args.all_tags,
                 recursive=self.args.recursive,
@@ -121,7 +126,10 @@ def add_parser(subparsers, parent_parser):
         formatter_class=argparse.RawDescriptionHelpFormatter,
     )
     metrics_show_parser.add_argument(
-        "path", nargs="?", help="Path to a metric file or a directory."
+        "targets",
+        nargs="*",
+        help="Metric files or directories (see -R) to show "
+        "(leave empty to display all)",
     )
     metrics_show_parser.add_argument(
         "-t",
@@ -155,8 +163,8 @@ def add_parser(subparsers, parent_parser):
         action="store_true",
         default=False,
         help=(
-            "If path is a directory, recursively search and process metric "
-            "files in path."
+            "If any target is a directory, recursively search and process "
+            "metric files."
         ),
     )
     metrics_show_parser.set_defaults(func=CmdMetricsShow)
diff --git a/dvc/exceptions.py b/dvc/exceptions.py
--- a/dvc/exceptions.py
+++ b/dvc/exceptions.py
@@ -196,10 +196,11 @@ def __init__(self, dvc_file_name, ex):
 
 
 class BadMetricError(DvcException):
-    def __init__(self, path):
+    def __init__(self, paths):
         super(BadMetricError, self).__init__(
-            "'{}' does not exist, not a metric or is malformed".format(
-                relpath(path)
+            "the following metrics do not exists, "
+            "are not metric files or are malformed: {paths}".format(
+                paths=", ".join("'{}'".format(path) for path in paths)
             )
         )
 
diff --git a/dvc/repo/metrics/show.py b/dvc/repo/metrics/show.py
--- a/dvc/repo/metrics/show.py
+++ b/dvc/repo/metrics/show.py
@@ -8,7 +8,7 @@
 
 from jsonpath_ng.ext import parse
 
-from dvc.exceptions import OutputNotFoundError, BadMetricError, NoMetricsError
+from dvc.exceptions import OutputNotFoundError, NoMetricsError
 from dvc.utils.compat import builtin_str, open, StringIO, csv_reader
 
 NO_METRICS_FILE_AT_REFERENCE_WARNING = (
@@ -159,8 +159,8 @@ def _collect_metrics(repo, path, recursive, typ, xpath, branch):
         path (str): Path to a metric file or a directory.
         recursive (bool): If path is a directory, do a recursive search for
             metrics on the given path.
-        typ (str): The type of metric to search for, could be one of the
-            following (raw|json|tsv|htsv|csv|hcsv).
+        typ (str): The type that will be used to interpret the metric file,
+            one of the followings - (raw|json|tsv|htsv|csv|hcsv).
         xpath (str): Path to search for.
         branch (str): Branch to look up for metrics.
 
@@ -259,7 +259,7 @@ def _read_metrics(repo, metrics, branch):
 
 def show(
     repo,
-    path=None,
+    targets=None,
     typ=None,
     xpath=None,
     all_branches=False,
@@ -267,16 +267,34 @@ def show(
     recursive=False,
 ):
     res = {}
+    found = set()
+
+    if not targets:
+        # Iterate once to call `_collect_metrics` on all the stages
+        targets = [None]
 
     for branch in repo.brancher(all_branches=all_branches, all_tags=all_tags):
-        entries = _collect_metrics(repo, path, recursive, typ, xpath, branch)
-        metrics = _read_metrics(repo, entries, branch)
+        metrics = {}
+
+        for target in targets:
+            entries = _collect_metrics(
+                repo, target, recursive, typ, xpath, branch
+            )
+            metric = _read_metrics(repo, entries, branch)
+
+            if metric:
+                found.add(target)
+                metrics.update(metric)
+
         if metrics:
             res[branch] = metrics
 
-    if not res:
-        if path:
-            raise BadMetricError(path)
+    if not res and not any(targets):
         raise NoMetricsError()
 
+    missing = set(targets) - found
+
+    if missing:
+        res[None] = missing
+
     return res

what's the purpose of doing `dvc metrics show` for a single path instead of using `cat`?

`cat ml_model/level_{1,2}/scores.json` ?
what about `dvc metrics show ml_model/` ?

related: https://github.com/iterative/dvc/issues/2318
To run it with -a or -T for example?
Or `metrics show -R dir` another example for a single target case
Indeed, @shcheklein :+1: 
diff --git a/tests/func/test_metrics.py b/tests/func/test_metrics.py
--- a/tests/func/test_metrics.py
+++ b/tests/func/test_metrics.py
@@ -7,7 +7,7 @@
 
 from dvc.repo import Repo as DvcRepo
 from dvc.main import main
-from dvc.exceptions import DvcException, BadMetricError, NoMetricsError
+from dvc.exceptions import DvcException, NoMetricsError
 from dvc.repo.metrics.show import NO_METRICS_FILE_AT_REFERENCE_WARNING
 from dvc.stage import Stage
 from dvc.utils import relpath
@@ -85,14 +85,14 @@ def setUp(self):
 
 class TestMetrics(TestMetricsBase):
     def test_show(self):
-        ret = self.dvc.metrics.show("metric", all_branches=True)
+        ret = self.dvc.metrics.show(["metric"], all_branches=True)
         self.assertEqual(len(ret), 3)
         self.assertEqual(ret["foo"]["metric"], "foo")
         self.assertEqual(ret["bar"]["metric"], "bar")
         self.assertEqual(ret["baz"]["metric"], "baz")
 
         ret = self.dvc.metrics.show(
-            "metric_json", typ="json", xpath="branch", all_branches=True
+            ["metric_json"], typ="json", xpath="branch", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         self.assertSequenceEqual(ret["foo"]["metric_json"], ["foo"])
@@ -100,7 +100,7 @@ def test_show(self):
         self.assertSequenceEqual(ret["baz"]["metric_json"], ["baz"])
 
         ret = self.dvc.metrics.show(
-            "metric_tsv", typ="tsv", xpath="0,0", all_branches=True
+            ["metric_tsv"], typ="tsv", xpath="0,0", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         self.assertSequenceEqual(ret["foo"]["metric_tsv"], ["foo"])
@@ -108,7 +108,7 @@ def test_show(self):
         self.assertSequenceEqual(ret["baz"]["metric_tsv"], ["baz"])
 
         ret = self.dvc.metrics.show(
-            "metric_htsv", typ="htsv", xpath="0,branch", all_branches=True
+            ["metric_htsv"], typ="htsv", xpath="0,branch", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         self.assertSequenceEqual(ret["foo"]["metric_htsv"], ["foo"])
@@ -116,7 +116,7 @@ def test_show(self):
         self.assertSequenceEqual(ret["baz"]["metric_htsv"], ["baz"])
 
         ret = self.dvc.metrics.show(
-            "metric_csv", typ="csv", xpath="0,0", all_branches=True
+            ["metric_csv"], typ="csv", xpath="0,0", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         self.assertSequenceEqual(ret["foo"]["metric_csv"], ["foo"])
@@ -124,7 +124,7 @@ def test_show(self):
         self.assertSequenceEqual(ret["baz"]["metric_csv"], ["baz"])
 
         ret = self.dvc.metrics.show(
-            "metric_hcsv", typ="hcsv", xpath="0,branch", all_branches=True
+            ["metric_hcsv"], typ="hcsv", xpath="0,branch", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         self.assertSequenceEqual(ret["foo"]["metric_hcsv"], ["foo"])
@@ -132,7 +132,7 @@ def test_show(self):
         self.assertSequenceEqual(ret["baz"]["metric_hcsv"], ["baz"])
 
         ret = self.dvc.metrics.show(
-            "metric_json_ext",
+            ["metric_json_ext"],
             typ="json",
             xpath="$.metrics[?(@.deviation_mse<0.30) & (@.value_mse>0.4)]",
             all_branches=True,
@@ -153,7 +153,7 @@ def test_show(self):
 
     def test_unknown_type_ignored(self):
         ret = self.dvc.metrics.show(
-            "metric_hcsv", typ="unknown", xpath="0,branch", all_branches=True
+            ["metric_hcsv"], typ="unknown", xpath="0,branch", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -161,7 +161,7 @@ def test_unknown_type_ignored(self):
 
     def test_type_case_normalized(self):
         ret = self.dvc.metrics.show(
-            "metric_hcsv", typ=" hCSV ", xpath="0,branch", all_branches=True
+            ["metric_hcsv"], typ=" hCSV ", xpath="0,branch", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -169,7 +169,7 @@ def test_type_case_normalized(self):
 
     def test_xpath_is_empty(self):
         ret = self.dvc.metrics.show(
-            "metric_json", typ="json", xpath="", all_branches=True
+            ["metric_json"], typ="json", xpath="", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -177,7 +177,7 @@ def test_xpath_is_empty(self):
 
     def test_xpath_is_none(self):
         ret = self.dvc.metrics.show(
-            "metric_json", typ="json", xpath=None, all_branches=True
+            ["metric_json"], typ="json", xpath=None, all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -185,7 +185,7 @@ def test_xpath_is_none(self):
 
     def test_xpath_all_columns(self):
         ret = self.dvc.metrics.show(
-            "metric_hcsv", typ="hcsv ", xpath="0,", all_branches=True
+            ["metric_hcsv"], typ="hcsv ", xpath="0,", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -193,7 +193,7 @@ def test_xpath_all_columns(self):
 
     def test_xpath_all_rows(self):
         ret = self.dvc.metrics.show(
-            "metric_csv", typ="csv", xpath=",0", all_branches=True
+            ["metric_csv"], typ="csv", xpath=",0", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -201,7 +201,7 @@ def test_xpath_all_rows(self):
 
     def test_xpath_all(self):
         ret = self.dvc.metrics.show(
-            "metric_csv", typ="csv", xpath=",", all_branches=True
+            ["metric_csv"], typ="csv", xpath=",", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -209,7 +209,7 @@ def test_xpath_all(self):
 
     def test_xpath_all_with_header(self):
         ret = self.dvc.metrics.show(
-            "metric_hcsv", typ="hcsv", xpath=",", all_branches=True
+            ["metric_hcsv"], typ="hcsv", xpath=",", all_branches=True
         )
         self.assertEqual(len(ret), 3)
         for b in ["foo", "bar", "baz"]:
@@ -412,11 +412,13 @@ def setUp(self):
         self.dvc.scm.checkout("master")
 
     def test(self):
-        with self.assertRaises(BadMetricError):
-            self.dvc.metrics.show("nested", all_branches=True, recursive=False)
+        ret = self.dvc.metrics.show(
+            ["nested"], all_branches=True, recursive=False
+        )
+        self.assertEqual(len(ret), 1)
 
         ret = self.dvc.metrics.show(
-            "nested", all_branches=True, recursive=True
+            ["nested"], all_branches=True, recursive=True
         )
         self.assertEqual(len(ret), 1)
         self.assertEqual(
@@ -697,7 +699,7 @@ def _test_metrics(self, func):
         self.dvc = DvcRepo(".")
 
         res = self.dvc.metrics.show(
-            "metrics.json", all_branches=True, typ="json", xpath="metrics"
+            ["metrics.json"], all_branches=True, typ="json", xpath="metrics"
         )
 
         self.assertEqual(
@@ -711,7 +713,7 @@ def _test_metrics(self, func):
         )
 
         res = self.dvc.metrics.show(
-            "", all_branches=True, typ="json", xpath="metrics"
+            all_branches=True, typ="json", xpath="metrics"
         )
 
         self.assertEqual(
@@ -777,7 +779,9 @@ def test_show(self):
             self._do_show(file_name, xpath)
 
     def _do_show(self, file_name, xpath):
-        ret = self.dvc.metrics.show(file_name, xpath=xpath, all_branches=True)
+        ret = self.dvc.metrics.show(
+            [file_name], xpath=xpath, all_branches=True
+        )
         self.assertEqual(len(ret), 3)
         for branch in self.branches:
             if isinstance(ret[branch][file_name], list):
@@ -844,3 +848,41 @@ def test_show_xpath_should_override_stage_xpath(dvc_repo):
 
     result = dvc_repo.metrics.show(xpath="m1")
     assert result == {"": {metric_file: [0.1]}}
+
+
+def test_show_multiple_outputs(dvc_repo, caplog):
+    with open("1.json", "w") as fobj:
+        json.dump({"AUC": 1}, fobj)
+
+    with open("2.json", "w") as fobj:
+        json.dump({"AUC": 2}, fobj)
+
+    os.mkdir("metrics")
+    with open("metrics/3.json", "w") as fobj:
+        json.dump({"AUC": 3}, fobj)
+
+    dvc_repo.run(cmd="", overwrite=True, metrics=["1.json"])
+    dvc_repo.run(cmd="", overwrite=True, metrics=["2.json"])
+    dvc_repo.run(cmd="", overwrite=True, metrics=["metrics/3.json"])
+
+    with caplog.at_level(logging.INFO, logger="dvc"):
+        assert 0 == main(["metrics", "show", "1.json", "2.json"])
+        assert '1.json: {"AUC": 1}' in caplog.text
+        assert '2.json: {"AUC": 2}' in caplog.text
+
+    caplog.clear()
+
+    with caplog.at_level(logging.INFO, logger="dvc"):
+        assert 0 == main(["metrics", "show", "-R", "1.json", "metrics"])
+        assert '1.json: {"AUC": 1}' in caplog.text
+        assert '3.json: {"AUC": 3}' in caplog.text
+
+    caplog.clear()
+
+    with caplog.at_level(logging.INFO, logger="dvc"):
+        assert 1 == main(["metrics", "show", "1.json", "not-found"])
+        assert '1.json: {"AUC": 1}' in caplog.text
+        assert (
+            "the following metrics do not exists, "
+            "are not metric files or are malformed: 'not-found'"
+        ) in caplog.text

metrics: show: support multiple targets
Currently, it only supports one single target.

From Al on discord:

```
this works fine: dvc metrics show ml_model/level_1/scores.json
this doesn't: dvc metrics show ml_model/level_{1,2}/scores.json
Error: unrecognized arguments: ml_model/level_2/scores.json
```

tests/func/test_metrics.py::TestMetrics::test_show tests/func/test_metrics.py::TestMetrics::test_type_case_normalized tests/func/test_metrics.py::TestMetrics::test_unknown_type_ignored tests/func/test_metrics.py::TestMetrics::test_xpath_all tests/func/test_metrics.py::TestMetrics::test_xpath_all_columns tests/func/test_metrics.py::TestMetrics::test_xpath_all_rows tests/func/test_metrics.py::TestMetrics::test_xpath_all_with_header tests/func/test_metrics.py::TestMetrics::test_xpath_is_empty tests/func/test_metrics.py::TestMetrics::test_xpath_is_none tests/func/test_metrics.py::TestMetricsRecursive::test tests/func/test_metrics.py::TestCachedMetrics::test_add tests/func/test_metrics.py::TestCachedMetrics::test_run tests/func/test_metrics.py::TestMetricsType::test_show tests/func/test_metrics.py::test_show_multiple_outputs
tests/func/test_metrics.py::TestMetrics::test_formatted_output tests/func/test_metrics.py::TestMetrics::test_show_all_should_be_current_dir_agnostic tests/func/test_metrics.py::TestMetricsReproCLI::test tests/func/test_metrics.py::TestMetricsReproCLI::test_binary tests/func/test_metrics.py::TestMetricsReproCLI::test_dir tests/func/test_metrics.py::TestMetricsCLI::test tests/func/test_metrics.py::TestMetricsCLI::test_binary tests/func/test_metrics.py::TestMetricsCLI::test_dir tests/func/test_metrics.py::TestMetricsCLI::test_non_existing tests/func/test_metrics.py::TestMetricsCLI::test_wrong_type_add tests/func/test_metrics.py::TestMetricsCLI::test_wrong_type_modify tests/func/test_metrics.py::TestMetricsCLI::test_wrong_type_show tests/func/test_metrics.py::TestNoMetrics::test tests/func/test_metrics.py::TestNoMetrics::test_cli tests/func/test_metrics.py::TestShouldDisplayMetricsEvenIfMetricIsMissing::test tests/func/test_metrics.py::test_show_xpath_should_override_stage_xpath