release-23.2: obs: export metrics about Go GC Assist work; add chart

[nicktrav]

Backport:

• 1/1 commits from "obs: export metrics about Go GC Assist work" (obs: export metrics about Go GC Assist work #118875)
• 1/1 commits from "ui: add charts on gc assist metric" (ui: add charts on gc assist metric #119506)

Please see individual PRs for details.

/cc @cockroachdb/release

---

Release note: None.

Release justification: Low risk observability change.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[nvanbenschoten]

did you manually confirm that this works when compiled with v23.2 and its version of Go? The runtime/metrics is unfortunately weakly typed and won't complain about unsupported metrics, it will just populate values with KindBad when sampling at runtime.

Reviewed 2 of 2 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @kyle-a-wong)

[nicktrav]

did you manually confirm that this works when compiled with v23.2 and its version of Go?

Here's a look at this, running locally for around 10 mins of a KV50 run. I assume that the proof is in the pudding - we see the GC assist timeseries on the new chart.

[nvanbenschoten]

I assume that the proof is in the pudding - we see the GC assist timeseries on the new chart.

Yes, that proves it. Thanks for checking.

[nicktrav]

Adding a note here that this technically adds new functionality, but there is precedence for backporting obs changes, and we're ok with that.