release-24.2: kvcoord: add a metric for number of partial batches in progress

[blathers-crl[bot]]

Backport 1/1 commits from #134092 on behalf of @yuzefovich.

/cc @cockroachdb/release

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It has been shown that adjusting kv.dist_sender.concurrency_limit cluster setting can improve the performance significantly. We currently already have two relevant counters:

• distsender.batches.async.sent tracks the total number of partial KV batches that have been executed in parallel since the node restart
• distsender.batches.async.throttled is the total number of times we tried to send a partial KV batch but couldn’t because we exhausted the kv.dist_sender.concurrency_limit.

This commit additionally introduces distsender.batches.async.in_progress gauge to indicate how many partial batch are being executed asynchronously at any point in time.

Fixes: #131125.

Release note: None

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Release justification: low-risk observability improvement.

[blathers-crl[bot]]

Thanks for opening a backport.

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[blathers-crl[bot]]

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?

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[cockroach-teamcity]

This change is