kvserver/rangefeed: add metrics for lockedMuxStream.Send calls

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Adds observability into the latency of lockedMuxStream.Send with a histogram metric that uses the IOLatencyBuckets bucket configuration (see https://github.com/cockroachdb/cockroach/pull/147440#discussion_r2112420050).

Also adds a counter for the number of slow sends, using the same criteria as the log line added in https://github.com/cockroachdb/cockroach/pull/146765.

Resolves: https://github.com/cockroachdb/cockroach/issues/147435 Epic: CRDB-51061 Release note: None

[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 

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bors r=wenyihu6

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