[WIP] metric: implement static labeling system

[dhartunian]

On the left: the metrics you know and love in DB Console
On the right: prometheus endpoint with same metrics split by label.

This is the minimum viable implementation that enables this. Some challenges to note:

• Raw manipulation of Metadata.Name is very problematic. There are other parts of the codebase that do this.
• When combining metrics into a single name w/ labels, it's crucial to make sure that the sum of the labeled metrics rolls up into a coherent thing. It's easy to combine, say, two different counts into a single metric where the sum is nonsense.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[dhartunian]

Closing in favor of #143511 which will merge this work.