CNDB-10907: Implement overlap diagnostics

[blambov]

What is the issue

Extra diagnostics methods to better understand UCS compaction hierarchy structure.

For Cassandra itself this provides a new --overlaps (-O) option to nodetool compactionstats that generates an overlap map showing the maximum overlaps for every shard of every level in the strategy, as well as a total for all sstables in the system (not necessarily the sum of the per-level overlaps).

Also implements methods to be used by CNDB-side diagnostics.

What does this PR fix and why was it fixed

...

Checklist before you submit for review

• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits

[eolivelli]

it would be great to expose this in CNDB, it should be pretty simple, it is only a matter of exposing the API in the REST API of the CompactorLeader

[blambov]

Sample printout:

Max overlap map for density.blobs:
  all: 26, per shard: [11, 11, 17, 17, 14, 14, 20, 20, 17, 17, 23, 23, 20, 20, 26, 26]
  unrepaired:0: 18, per shard: [6, 10, 14, 18]
  unrepaired:1: 9, per shard: [6, 9, 5, 8, 4, 7, 3, 6]
  unrepaired:2: 3, per shard: [3, 3, 3, 3, 3, 3, 2, 2, 3, 3, 3, 3, 3, 3, 2, 2, 3, 3, 3, 3, 3, 3, 2, 2, 3, 3, 3, 3, 3, 3, 2, 2]

One thing this makes very visible is the effect of early open: L0 overlaps are small for the lower keys because ongoing compactions have already early-opened the result. L1 also has this (every second overlap is higher).

[sonarqubecloud]

Quality Gate failed

Failed conditions
70.2% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[cassci-bot]

✔️ Build ds-cassandra-pr-gate/PR-1453 approved by Butler

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Approved by Butler
See build details here