Summary:
**Context/Summary:**
This PR adds new stats to measure compaction readahead size for rocksdb managed prefetching (not FS prefetching). It can be used to verify compaction read-ahead is doing what's configured. This PR also excludes compaction readahead stats from user scan readahead stats measured in existing stats so there is a cleaner separating between these two.
Bonus: this PR also included some typo fixing about "io activities"
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13520
Test Plan: Modified existing test to verify stats
Reviewed By: archang19
Differential Revision: D72892850
Pulled By: hx235
fbshipit-source-id: 1a73182061baa044c9c9193a2b0fd967ffe75c4a