[SnapshotV2] Add timestamp of last successful fetch of pinned timestamps in node stats

[ltaragi]

Description

This change adds the timestamp for the last successful fetch of pinned timestamps to the node stats:

• We make use of the static method getPinnedTimestamps() of RemoteStorePinnedTimestampService. The first value of the returned tuple is when the last fetch of pinned timestamps is triggered . This is the timestamp we need to add to node stats.
• We have ensured backward compatibility with version checks
• We have added a new remote store specific metric class RemoteStoreNodeStats in the node stats

Related Issues

Resolves #15896

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 7500c9d: null

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❌ Gradle check result for 42a1030: FAILURE

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❌ Gradle check result for a617c18: FAILURE

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[github-actions]

✅ Gradle check result for b238922: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 48.57143% with 18 lines in your changes missing coverage. Please review.

Project coverage is 71.98%. Comparing base (d131d58) to head (b0a71d2).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...nsearch/node/remotestore/RemoteStoreNodeStats.java	45.00%	8 Missing and 3 partials ⚠️
...rch/action/admin/cluster/node/stats/NodeStats.java	44.44%	3 Missing and 2 partials ⚠️
...src/main/java/org/opensearch/node/NodeService.java	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #15611      +/-   ##
============================================
+ Coverage     71.89%   71.98%   +0.09%     
- Complexity    64466    64528      +62     
============================================
  Files          5288     5289       +1     
  Lines        301439   301471      +32     
  Branches      43552    43558       +6     
============================================
+ Hits         216706   217011     +305     
+ Misses        66931    66676     -255     
+ Partials      17802    17784      -18     

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[github-actions]

❌ Gradle check result for 7f81828: FAILURE

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[github-actions]

❌ Gradle check result for 7fae8db: FAILURE

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[ltaragi]

❌ Gradle check result for 7fae8db: FAILURE

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Flaky test: #14316

[github-actions]

✅ Gradle check result for 4e42331: SUCCESS

[github-actions]

✅ Gradle check result for b0a71d2: SUCCESS

[dblock]

This needed an API spec change as remote_store is newly returned. Hitting it in opensearch-project/opensearch-api-specification#643.