[Backport 2.x] [Refactor] Metadata members from ImmutableOpenMap to j.u.Map (#7165)

[nknize]

Backport 1ab22e1 from #7165.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/18281/
• CommitID: 802476f
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/18284/
• CommitID: 89bc467
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/18290/
• CommitID: ba756e7
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.search.SearchWeightedRoutingIT.testStrictWeightedRoutingWithCustomString
      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testPressureServiceStats
      1 org.opensearch.indices.replication.SegmentReplicationTargetServiceTests.testStartReplicationListenerSuccess

• URL: https://build.ci.opensearch.org/job/gradle-check/18296/
• CommitID: ba756e7
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Merging #8241 (ba756e7) into 2.x (e3d8f7d) will decrease coverage by 0.12%.
The diff coverage is 63.78%.

@@             Coverage Diff              @@
##                2.x    #8241      +/-   ##
============================================
- Coverage     70.72%   70.61%   -0.12%     
+ Complexity    56865    56842      -23     
============================================
  Files          4703     4705       +2     
  Lines        269109   269154      +45     
  Branches      39778    39785       +7     
============================================
- Hits         190333   190068     -265     
- Misses        62468    62774     +306     
- Partials      16308    16312       +4     

Impacted Files	Coverage Δ
...rg/opensearch/gradle/CheckCompatibilityTask.groovy	0.00% <0.00%> (ø)
...main/java/org/opensearch/common/util/set/Sets.java	72.09% <ø> (ø)
...ing/delete/TransportDeleteDanglingIndexAction.java	9.58% <0.00%> (ø)
...g/opensearch/cluster/coordination/Coordinator.java	79.13% <ø> (+0.23%)	⬆️
...rch/cluster/coordination/RemoveCustomsCommand.java	0.00% <0.00%> (ø)
...dination/UnsafeBootstrapClusterManagerCommand.java	0.00% <0.00%> (ø)
.../src/main/java/org/opensearch/gateway/Gateway.java	8.33% <0.00%> (ø)
...h/index/shard/RemoveCorruptedShardDataCommand.java	83.89% <ø> (+3.06%)	⬆️
...pensearch/rest/action/cat/RestTemplatesAction.java	22.44% <0.00%> (+0.44%)	⬆️
.../java/org/opensearch/snapshots/RestoreService.java	55.78% <0.00%> (+1.57%)	⬆️
... and 27 more

... and 445 files with indirect coverage changes