release-21.2: kvserver: fix nil pointer dereferencing in replicaRankings.topQPS

[blathers-crl]

Backport 1/1 commits from #74507 on behalf of @erikgrinaker.

/cc @cockroachdb/release

---

If replicaRankings.topQPS() was called before a QPS accumulator had
been set for the rankings, it would attempt to dereference a nil pointer
and panic. This could happen e.g. if the status server's
/_status/hotranges endpoint was accessed during node startup.

The initialization story here is a bit unclear -- it appears that the
accumulator is set up in Store.Capacity(), which is called by
Store.Descriptor(). So this will only be initialized once the store
descriptor is requested by someone, e.g. during Store.GossipStore() or
MetricsRecorder.GenerateNodeStatus(). I haven't made an attempt at
reviewing or revising this.

This patch explicitly checks if a QPS accumulator has been set before
attempting to access it, avoiding the nil pointer dereferencing.

Release note (bug fix): Fixed a panic when attempting to access the
hottest ranges (e.g. via the /_status/hotranges endpoint) before
initial statistics had been gathered.

---

Release justification: fixes a node crash.

[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?

[cockroach-teamcity]

This change is