Metrics: creating tags with fallback value

[asanderson15]

We frequently want to create a tag with a known key, but potentially invalid value, in which case we want to fall back to a known value such as the string unknown. This allows us to do so without repeatedly rewriting the same fallback code.

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This change is 

[bmoylan]

Will give @nmiyake a chance to take a look but we'll get a release out soon

[nmiyake]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @asanderson15)

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metrics/tag.go, line 80 at r2 (raw file):

// NewTagWithFallbackValue returns the result of calling NewTag, and if that fails, calls MustNewTag with a fallback
// value. Note: because MustNewTag will panic if it fails, both the key and fallback value must be known to be valid.

Let's expand this comment to describe the intended use case:

This function is useful when value is provided as a runtime input and the desired behavior is to fall back to use a known valid value (for example, "unknown") when the value is invalid.

[nmiyake]

Sorry for dragging on this -- it's because this PR exists in the zone where I don't love adding it to the API, but understand the current pain/code duplication and can't say that it doesn't make sense to add out-right :)

At the end of the day, I don't see a major down-side to adding this, so OK with it on the whole. Left some feedback on preference to update the documentation a bit, but otherwise I'm good with this.

[asanderson15]

No problem @nmiyake. That all makes sense. Agree with the comment and added the add'l docs.