[Backport 2.x] Change InternalSignificantTerms to only sum shard level counts in final reduce (#8735)

[jed326]

Backport PR for #8735

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/20624/
• CommitID: 86d577e

[jed326]

@andrross here is the backport PR for #8735. Thanks!

[codecov]

Codecov Report

Merging #8806 (86d577e) into 2.x (d83147f) will decrease coverage by 0.19%.
The diff coverage is 64.04%.

@@             Coverage Diff              @@
##                2.x    #8806      +/-   ##
============================================
- Coverage     70.80%   70.61%   -0.19%     
+ Complexity    57405    57309      -96     
============================================
  Files          4759     4758       -1     
  Lines        271578   271488      -90     
  Branches      40034    40020      -14     
============================================
- Hits         192283   191716     -567     
- Misses        62795    63307     +512     
+ Partials      16500    16465      -35     

Impacted Files	Coverage Δ
...cluster/reroute/TransportClusterRerouteAction.java	46.05% <0.00%> (+1.31%)	⬆️
...ices/shards/TransportIndicesShardStoresAction.java	4.90% <0.00%> (ø)
...a/org/opensearch/cluster/routing/RoutingTable.java	94.37% <ø> (ø)
...on/lucene/search/function/RandomScoreFunction.java	68.96% <ø> (ø)
...java/org/opensearch/common/recycler/Recyclers.java	91.42% <ø> (ø)
...g/opensearch/common/util/AbstractPagedHashMap.java	71.05% <ø> (ø)
.../java/org/opensearch/common/util/BytesRefHash.java	87.87% <ø> (ø)
.../java/org/opensearch/common/util/LongLongHash.java	92.59% <ø> (ø)
...g/opensearch/common/util/ReorganizingLongHash.java	95.31% <ø> (ø)
.../src/main/java/org/opensearch/gateway/Gateway.java	8.33% <0.00%> (ø)
... and 52 more

... and 468 files with indirect coverage changes