feat: add groupby metrics to prometheus emitter

[TessaIO]

Fixes #17908.

Description

Add Groupby metrics to Prometheus emitter

Release note

---

Add Groupby metrics to Prometheus emitter

---

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[TessaIO]

cc @gianm

[gianm]

LGTM, thanks!

Note for possible future work: similar changes could likely be made to the statsd, opentsdb, etc emitters, or we could unify the configs. Many emitters have some way of defining what metrics are emitted using JSON files, although the default versions of those JSON files haven't yet been unified. (You don't have to do this, I'm just writing it in case anyone looks and wonders about other emitters.)