WiP stats: add stats sink speed test with histogram updater to try to repro #37831

[jmarantz]

Commit Message: Repro of contention bottleneck (#37831) in stats sinks with lots of threads, and a possibly remedy, implemented within a test sink.

Note that in its current form there are several issues:

• the sample remedy has a bug -- it holds a pointer in its map to a histogram which might not survive a config update, so we need to take a reference to it. This is possible (the ref-count is internal) but is not made convenient via the API currently.
• the remedy still has a contention bottleneck on the shared map, it's just a little better than serialzing the names under lock. But we don't need to contend; we could add another TLS object. This needs to be designed.
• The remedy is implemented in a single sink, and should probably be shared across any sinks that require capturing all the histogram updates. This is not needed for sinks where the aggregated histogram data csn be used instead, thus such sinks should not need to pay for any new buffering. We probably need two Sink variants: one that provides per-change histogram updates, and one that only provides aggregated histogram data in the snapshot.

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Caused by: #37859 was opened by jmarantz.

see: more, trace.