Graduate native histogram ingestion and close the v1-guarantees gaps

[CharlieTLe]

Groundwork for v1.22.0. Refs #5922.

Graduate native histogram ingestion

-blocks-storage.tsdb.enable-native-histograms shipped as experimental in 1.18.0
(2024-09-03) — four minors and roughly two years of exposure by 1.22.0.
docs/guides/native-histograms.md already contains zero "experimental" language, which
directly contradicted v1-guarantees.md.

The flag name does not change, so there is no deprecation shim. This does not touch
-blocks-storage.tsdb.out-of-order-cap-max, which has its own bullet.

This cycle's native-histogram fixes are consumer-side decode/merge, not storage-format:
#7645 (panic on zero-count float histograms) and #7555 (ordering in split-by-interval
merges) are both fixed and tested; #7636 and #7569 are additive observability.

Experimental flags that never got a v1-guarantees.md entry

• -distributor.num-query-workers (feat(distributor): add goroutine worker pool for query fan-out to ing… #7623) — marked EXPERIMENTAL: in its flag help.
• The entire Thanos PromQL engine block. -querier.thanos-engine says "Experimental."
  in its help, yet nothing from pkg/engine/config.go was listed. Added as one grouped
  bullet covering -querier/-ruler .thanos-engine, .enable-x-functions,
  .optimizers, .decoding-concurrency, .selector-batch-size.
• The entire parquet flag surface. Only -parquet-converter.max-num-columns was
  listed. Grouped by prefix rather than enumerated, since the labels-cache and
  row-ranges-cache flags are pure boilerplate.
• -querier.query-protection.rejection (Implement Resource Based Throttling on Querier #7442) added to the existing query-rejection
  bullet, and -querier.query-protection.eviction.max-evictions-per-cycle (Add Query Resource Based Eviction #7488) to
  the eviction bullet.

Deliberately not graduated

• Parquet. docs/proposals/parquet-storage.md still says Status: Proposed and
  docs/guides/parquet-mode.md still warns it is experimental. Six substantive PRs landed
  this cycle, including Support sharded Parquet file querying and conversion #7610 (sharded conversion — a file-format change) and feat(parquetconverter): Add max-num-columns config for automatic parquet sharding #7624.
• Tenant-federation regex matcher. Only two minors of exposure, pkg/cortex/modules.go
  still calls WarnExperimentalUse, and include resolved tenant fingerprint in result cache keys to prevent stale data after tenant changes #7562 fixed the result cache returning stale
  cross-tenant data this cycle. Worth revisiting in 1.23.0.

Audit outcome: two FEATUREs that look missing but are not

I checked all 14 of this cycle's FEATUREs. #7374 (timeout classification) and #7385
(-memberlist.cluster-label) have no experimental marker anywhere — no EXPERIMENTAL:
in the flag help, no WarnExperimentalUse, no mention in their changelog entries. Adding
them to v1-guarantees.md would retroactively remove guarantees from features that
shipped with them, so I left them out. Flagging it in case that was not the intent.

Older gaps found but left alone

Outside this cycle's scope, so not in this PR — worth a 1.23.0 follow-up alongside the
make check-experimental-flags lint idea: -distributor.num-push-workers, the ring
token-generation strategy, ruler ring replication factor / zone awareness, ruler rule
deduplication, and the expanded postings cache.