How would I find a UINavigationBar's current UIBarMetrics value?
<p>I'm using the <code>setBackButtonTitlePositionAdjustment:forBarMetrics:</code> method on UIBarButtonItem's appearance like so:</p>

<pre><code>UIBarButtonItem.appearance().setBackButtonTitlePositionAdjustment(UIOffsetMake(12, -1), forBarMetrics: .Default)
UIBarButtonItem.appearance().setBackButtonTitlePositionAdjustment(UIOffsetMake(15, -1), forBarMetrics: .Compact)
</code></pre>

<p>Then, in a view controller I'd like to use <code>backButtonTitlePositionAdjustment(for:)</code> to get the value. I assumed there must be a UIBarMetrics value I could use - perhaps in the navigation bar or a UITraitCollection, but I think I'm missing something obvious?</p>