release-23.1: kvclient,server: propagate pprof labels for BatchRequests

[blathers-crl]

Backport 2/2 commits from #101404 on behalf of @adityamaru.

/cc @cockroachdb/release

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This change teaches DistSender to populate a BatchRequest's header with the pprof labels set on the sender's context. If the node processing the request on the server side has CPU profiling with labels enabled, then the labels stored in the BatchRequest will be applied to the root context of the goroutine executing the request on the server. Doing so will ensure that all server-side CPU samples of the root goroutine and all its spawned goroutine will be labeled correctly.

Propagating these labels across RPC boundaries is useful to correlate server side samples with the sender. For example, in a CPU profile generated with this change, we will be able to identify which backup job sent an ExportRequest that is causing a CPU hotspot on a remote node.

Fixes: #100166
Release note: None

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Release justification: debugging improvement

[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 

[adityamaru]

merging this as we haven't noted any fallout