[Metrics] Fixed metrics with no required tags currently being blackhole'd

[alexeykudinkin]

Why are these changes needed?

Currently, Metric implementation requires you to specify tag_keys of the tags that have to be recorded. However it also does NOT allow you to record additional tags on top of the ones you have provided during creation.

This is problematic since it couples metric creation with its usage: you can't, say add new tags to metric usage w/o updating its definition (which might be in a separate module, library, etc)

One straightforward example: if you haven't specified any tag_keys during metric creation it's impossible for you to add any tags during recording.

Proposal:

Treat tag_keys as only a set of required tags (ie allow additional tags to be specified during recording)

Related issue number

N/A

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