[TraceQL Metrics] API definitions and plumbing

[zalegrala]

What this PR does:

This is the result of a collaboration effort and requires #3227 for the underlying work of providing the data. Here we build out the API for metrics in Tempo.

• Update API and proto definitions
• Build out the generator API
• Wire up the generator API to queriers
• Handle the query and backend sharding on the frontend

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[mdisibio]

Closing in favor of #3252