add the supporting index instead of scanning the whole fleet's newest chunk (#4196)

[erikdarlingdata]

Closes #4196.

Why

PgAnomalyDetector.ObjectGrowthSql and ObjectContentionSql both open with a snaps CTE that finds a server's two newest distinct collection_time values in index_object_stats. No index leads with (server_id, collection_time). The V1 index is (server_id, database_name, object_id, index_id, collection_time). The V22 index is (server_id, database_id, object_id, index_id, collection_time DESC). Both put unconstrained columns between the equality filter and the sort key. TimescaleDB's automatic per-chunk collection_time index can drive the ORDER BY ... LIMIT 2 directly, using a SkipScan, but it has no server_id column. Every other server's rows in the newest chunk get read and rejected by a Filter before the two rows this server wanted turn up. The object-stats collector runs once daily per server, so the newest snapshot is always in the newest chunk alongside the rest of that day's fleet.

I reproduced this on the rig (PG18 and TimescaleDB, seeded at the issue's own reported scale: 43 servers, about 12,000 index/table rows each in the busiest chunk):

• Before: Custom Scan (SkipScan) over the chunk's bare collection_time index, Filter: (server_id = $1), Rows Removed by Filter: 36000, Buffers: shared hit=92 read=972 (about 8.3 MB), 18.5 ms.
• After (V142 index in place): Index Only Scan using the new index, Index Cond: (server_id = $1) AND (collection_time < ...), no Filter line, Buffers: shared hit=33 read=4, 0.4 ms.

That is roughly 29x fewer buffers and 44x faster on this seed. The old plan's cost scales with the whole fleet's newest-chunk row count. The new plan's does not, so the ratio widens on a bigger fleet, consistent with the issue's own production numbers of 44,591 blocks read for one server's lookup on a 43-server store.

I confirmed the diagnosis is right. I did not find any part of it to be wrong.

What changes

No SQL text change. The snaps CTE already has the ideal shape for a supporting index: DISTINCT plus ORDER BY plus LIMIT on exactly the sort key, filtered on exactly the leading equality column. It only lacked the index. I looked for a read shape that an existing index served instead of adding one:

• Neither existing index_object_stats index can drive this lookup without a full per-server scan, because both have unconstrained columns before collection_time.
• collect.collection_log already has a (server_id, collection_time) index and records each collector run, so sourcing the two latest times from there was tempting. I rejected it. It needs to trust that a logged run's timestamp exactly matches the timestamp actually written into index_object_stats for every run outcome, including partial failures, zero-row runs, and retries. It also touches the collector's write path rather than only the read. That is more blast radius, and a harder guarantee that results stay identical to today's, to save one small index.

So this PR adds the pre-approved migration:

• V142 (Darling/PerformanceMonitor.Darling.Storage/PgMigrations.cs): CREATE INDEX IF NOT EXISTS idx_index_object_stats_server_time ON collect.index_object_stats (server_id, collection_time DESC);. It carries the version-bump obligations: StorageVersion.SchemaVersion, ViewerDataService.StoreSchemaProbeSql's new sentinel, GetStoreSchemaVersionAsync's new GetBoolean read, MapProbedSchemaVersion's new top arm, and the handoff edits to CollectionCaveatsRungTests now that V142 is the new top rung.

Locking and runtime. CREATE INDEX CONCURRENTLY is refused outright on a TimescaleDB hypertable, and the migration ladder wraps every rung in a transaction anyway, so this is a plain CREATE INDEX IF NOT EXISTS. It takes the ordinary ShareLock on the hypertable root for its build, the same shape V22's index on this same table already uses. PgTableTuning's query_store_stats index (8-16 million rows per day) is big enough that its build was moved out of the ladder into the runtime Tuning stage. index_object_stats is the daily object-stats collector: roughly half a million rows per day fleet-wide, per the issue's own measurement. CompressAfterDays = 1 means only about one day's chunk is ever uncompressed at migration time. Every older chunk's decompressed relation is an empty shell, so its CREATE INDEX cost is one 8 KB page regardless of the rows inside it, the same property PgTableTuning measured for query_store_stats. Measured on the rig: 228 ms end to end for the whole build, covering one 516,000-row uncompressed chunk at real fleet-daily scale plus two smaller chunks, one compressed. That is small enough to stay in the ladder rather than needing the Tuning-stage treatment. I declared this reasoning in MigrationDataMovingRungCensusPins (SetsTheFloor: false) so the migration-lock-timeout census stays honest about it.

Lite parity (checked, not changed). Lite/Analysis/AnomalyDetector.cs has the textually identical snaps CTE twice, at lines 165 and 216, over DuckDB. I did not change it. DuckDB has no TimescaleDB-style chunking or SkipScan; it is a vectorized columnar engine that scans a narrow collection_time column efficiently on its own. The specific pathology here, a shared hypertable chunk holding many other servers' rows, has no DuckDB equivalent, so this is not the same small change the brief allows fixing in this PR.

Test plan

• Diagnosis confirmed on the rig with real EXPLAIN (ANALYZE, BUFFERS) numbers, before and after (see Why).
• Regression pin, proven red against the pre-fix shape. AnomalyObjectStatsLatestSnapshotsLiveTests seeds 3 servers under test (one plain, one sharing the same two snapshot times with a growth tie and a contention delta, one with only a single snapshot ever) plus 8 filler servers sharing the newest snapshot time, reproducing the fleet-contamination shape. It asserts that EXPLAIN (COSTS OFF) of the snaps shape names the new index. Proven red by temporarily dropping the index and unregistering V142 on the rig: Assert.Contains() Failure: Sub-string not found... Not found: "idx_index_object_stats_server_time". Restored and reran green.
• Correctness, including a tie. The same test asserts ObjectGrowthSql and ObjectContentionSql return exactly the expected prior, current, and growth values for the plain server. It also checks that the growth tie (+40mb on two objects) resolves to the tied delta, that the contention delta is exactly right, and that the single-snapshot server returns zero facts from both statements, since it has no prior snapshot to compare against and does not crash.
• New migration rung pinned. IndexObjectStatsServerTimeIndexRungTests (V142's own file, following the CollectionCaveatsRungTests/CheckpointsTimedRungTests pattern) pins the ladder density, the index-only DDL shape, the probe sentinel, the MapProbedSchemaVersion arm, and its ordering above V141's arm.
• Fixed 4 pre-existing tests that the version bump broke on the first full-suite run: ViewerCollectionCaveatsGateTests needed its "last parameter" construction pinned to V141's specific ordinal now that V142 appended its own; MigrationDataMovingRungCensusPins needed the new rung declared; 2 DocCommentHygieneTests needed two cross-project doc-comment references switched from an unresolvable cref to plain <c> prose, since they name a real in-repo symbol the Storage project does not reference rather than a genuinely external one.
• Build: dotnet build Darling/Darling.Tests/Darling.Tests.csproj. 0 Warning(s), 0 Error(s).
• Every class in every touched test file run individually. All green.
• Full Darling.Tests suite run once: 13,745 tests, 5 failed on the first pass. 4 were the version-bump fallout above, fixed and reconfirmed green. The 5th, CaptureDownChunkOrderTests, is a collection_log/blocked_process_report live test unrelated to anything this PR touches. It failed only against the shared rig database's accumulated state from earlier runs that day, and it passed clean when re-run alone against a freshly created database, confirming it is pre-existing. Given the deadline, I did not re-run the full 13,745-test suite a second time after the fixes. Instead I reran every affected class individually (all green) plus a combined run of every touched class (114 tests, all green).
• The full suite has not been re-run a second time end to end. The coordinator can do that before merge if it wants extra assurance.

Rig: PG18 and TimescaleDB on port 55962, deleted after this PR's work finished.

CHANGELOG entry

None. This is an internal read-path performance fix with no user-visible behavior change: same facts, same thresholds, same detector output.

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