Stats speed test

[jmarantz]

Description: adds a speed-test measuring the single-threaded performance of the stats store with and without tls. of course every machine is different, but on mine a typical run gives results similar to this:

bazel-bin/test/common/stats/thread_local_store_speed_test \
 --benchmark_report_aggregates_only=true --benchmark_repetitions=100
--------------------------------------------------------------
Benchmark                       Time           CPU Iterations
--------------------------------------------------------------
BM_StatsNoTls_mean       16249144 ns   16248983 ns         37
BM_StatsNoTls_median     16176011 ns   16175443 ns         37
BM_StatsNoTls_stddev       484905 ns     484909 ns         37
BM_StatsWithTls_mean     20958468 ns   20958244 ns         30
BM_StatsWithTls_median   20899829 ns   20899217 ns         30
BM_StatsWithTls_stddev     576702 ns     576784 ns         30

with variances in the single-digit percent range. This should help quantify the perf impact of potential changes to the stats system, to justify #4711 and #4715

Risk Level: low
Testing: only the new test
Docs Changes:
Release Notes:

[mattklein123]

Very nice!

[mattklein123]

Is it worth it to have a variant of this test that use shared memory? I'm guessing for most of the things we care about (cached fast path) it doesn't really matter, but possibly worth putting in a TODO like you did for the MT thread contention case below?

[jmarantz]

yes, good idea; done.