internal/observability: Adding metricProcedure field

[elliotchance]

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The metricProcedure replicates the behavior of a variety of internal libraries when converting the procedure for metrics (M3). This field is critical to being able to support log correlation with metrics. See ERD for full details.

https://docs.google.com/document/d/1B45wjZDQL0olCoENh3XYMyvCPI2A3hw8OeU2IhrtBtA

• [ ] Description and context for reviewers: one partner, one stranger
• [ ] Docs (package doc)

RELEASE NOTES:

New metricProcedure log field will contain M3-compatible lookup values for the YARPC procedure.

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[rabbbit]

Have we considered just making this a breaking change in logs?

Folks cannot setup alerts, which is convenient - nobody will get paged. There will be some minor chaos for a day or two, but I think I would stamp this - the goal would be to reduce the future confusion and noise.

[biosvs]

Have we considered just making this a breaking change in logs?

I think, it may be useful to have a yarpc-styled procedure name too. I mean, I personally use it all the time, though it might be a skew of library owner.