metric: remove mutex around windowed histogram update

[dhartunian]

Previously, we used an RWMutex to serialize access to the windowed histogram. This was done because there's a rotation that needs to happen that moves the currently active histogram into the prev variable and creates a fresh one to update in cur. This is done to maintain the window.

This change limits the mutex usage to rotation and window snapshotting, removing the need to take a read lock during updates since we can atomically grab the cur histogram.

There's a bit of nuance to manage around the fact that prev can be nil on the first iteration, since cur is the first histogram. After a single rotation, prev will not be nil again.

The prometheus histogram itself (what's stored in the atomic.Value) is thread-safe.

old:  https://github.com/dhartunian/cockroach/commit/507c68f8abb0c9a3f527c66cb42129c2cb096e7d Merge https://github.com/cockroachdb/cockroach/pull/140092
new:  https://github.com/dhartunian/cockroach/commit/ff9dd529682dbc6d485687e5451149c28bd82fd7 metric: remove mutex around windowed histogram upd
args: benchdiff "./pkg/util/metric" "-b" "-r" "BenchmarkHistogramRecordValue" "-c" "10" "--preview"

name                                     old time/op    new time/op    delta
HistogramRecordValue/insert_zero-10        30.7ns ± 8%    24.3ns ± 2%  -20.94%  (p=0.000 n=9+8)
HistogramRecordValue/insert_integers-10    37.4ns ± 6%    33.8ns ± 1%   -9.83%  (p=0.000 n=8+8)
HistogramRecordValue/random_integers-10    44.9ns ± 9%    41.7ns ± 1%   -7.25%  (p=0.000 n=8+8)

name                                     old alloc/op   new alloc/op   delta
HistogramRecordValue/insert_integers-10     0.00B          0.00B          ~     (all equal)
HistogramRecordValue/insert_zero-10         0.00B          0.00B          ~     (all equal)
HistogramRecordValue/random_integers-10     0.00B          0.00B          ~     (all equal)

name                                     old allocs/op  new allocs/op  delta
HistogramRecordValue/insert_integers-10      0.00           0.00          ~     (all equal)
HistogramRecordValue/insert_zero-10          0.00           0.00          ~     (all equal)
HistogramRecordValue/random_integers-10      0.00           0.00          ~     (all equal)

Part of #133306

Release note: None

[cockroach-teamcity]

This change is