release-23.1: kvserver,storepool: misc rebalance logging improvements

[blathers-crl]

Backport 2/2 commits from #102825 on behalf of @kvoli.

/cc @cockroachdb/release

---

The store list string returned the mean leases, ranges and
queries-per-second float values without limiting the number of decimal
places. This led to log lines with needlessly long decimals:

avg-ranges=40.66666666666667... avg-leases=10.166666666666666...

This PR updates the store list string formatting to 2 decimal
places for float values.

Previously, the easiest method of determining the current rebalance
objective from logs was to view the cluster setting and check for
logging indicating a mixed version cluster - this was cumbersome.

This PR annotates the ctx in the store rebalancer loop with an
additional tag: obj. The obj tag indicates the current rebalance
objective, either cpu or qps currently.

resolves: #102812

Release note: None

---

Release justification: Log line only changes which improve obs.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• 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.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[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 

[andrewbaptist]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @knz and @kvoli)

[kvoli]

TYFTR - I'll bake this for a bit before trying to merge.