Make Telemetry Tags Immutable (opensearch-project#20788)

Replace the mutable HashMap-backed Tags with an immutable design using
sorted parallel arrays and a precomputed hashCode. Adds allocation-efficient
factories (Tags.of, Tags.concat, Tags.fromMap, Tags.toMap), an EMPTY
singleton, and content-based equals/hashCode so Tags can be safely used as
map keys, stored in fields, and shared across threads.

Deprecated API (create(), addTag()) preserved for backward compatibility;
addTag() now returns a new instance instead of mutating in place. Fixes
AutoForceMergeMetrics which called addTag() without reassigning the return
value -- a silent no-op now that Tags is immutable.

---------

Signed-off-by: Sam Akrah <sakrah@uber.com>
Co-authored-by: Sam Akrah <sakrah@uber.com>

Author: sakrah

CHANGELOG.md

@@ -40,12 +40,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Added support of WarmerRefreshListener in NRTReplicationEngine to trigger warmer after replication on replica shards ([#20650](https://github.com/opensearch-project/OpenSearch/pull/20650))
 
 ### Changed
+- Make telemetry `Tags` immutable ([#20788](https://github.com/opensearch-project/OpenSearch/pull/20788))
 - Move Randomness from server to libs/common ([#20570](https://github.com/opensearch-project/OpenSearch/pull/20570))
 - Use env variable (OPENSEARCH_FIPS_MODE) to enable opensearch to run in FIPS enforced mode instead of checking for existence of bcFIPS jars ([#20625](https://github.com/opensearch-project/OpenSearch/pull/20625))
 - Update streaming flag to use search request context ([#20530](https://github.com/opensearch-project/OpenSearch/pull/20530))
 - Move pull-based ingestion classes from experimental to publicAPI ([#20704](https://github.com/opensearch-project/OpenSearch/pull/20704))
 
 ### Fixed
+- Fix `AutoForceMergeMetrics` silently dropping tags due to unreassigned `addTag()` return value ([#20788](https://github.com/opensearch-project/OpenSearch/pull/20788))
 - Fix flaky test failures in ShardsLimitAllocationDeciderIT ([#20375](https://github.com/opensearch-project/OpenSearch/pull/20375))
 - Prevent criteria update for context aware indices ([#20250](https://github.com/opensearch-project/OpenSearch/pull/20250))
 - Update EncryptedBlobContainer to adhere limits while listing blobs in specific sort order if wrapped blob container supports ([#20514](https://github.com/opensearch-project/OpenSearch/pull/20514))