* feat(1.x,file_store): port metrics for merge work
This commit ports metrics around merging tsm blocks when executing a
query. These will appear in EXPLAN ANALYZE results. The new information
records the time spent merging blocks, the number of blocks merged,
roughly the number of values merged into the first block of each
ReadBlock call, and the number of times that single calls to ReadBlock
have more than 4 block merges. The multiblock merge is sequential and
might benefit from a tree merge algorithm. The latter stat helps
identify if the engineering effort would be fruitful.
* closes#26614
* chore: switch to a timer for duration printing of times
* chore: rename method
* fix: avoid race and use new atomic primitive
4e8a3b389b