Fix unassigned ml system shard replicas

[b4sjoo]

Description

In previous ml system shard replicas always stays on an arbitrary number and cannot expand automatically, which will cause unassigned shards and yellow cluster problem. This PR can resolve this issue.

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

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Merging #1315 (596bb7c) into 2.x (104b1b7) will decrease coverage by 0.20%.
The diff coverage is 21.05%.

@@             Coverage Diff              @@
##                2.x    #1315      +/-   ##
============================================
- Coverage     79.40%   79.20%   -0.20%     
+ Complexity     2247     2245       -2     
============================================
  Files           188      188              
  Lines          9074     9089      +15     
  Branches        903      904       +1     
============================================
- Hits           7205     7199       -6     
- Misses         1461     1481      +20     
- Partials        408      409       +1     

Flag	Coverage Δ
ml-commons	79.20% <21.05%> (-0.20%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...va/org/opensearch/ml/indices/MLIndicesHandler.java	59.34% <21.05%> (-10.40%)	⬇️

... and 2 files with indirect coverage changes

[ylwu-amzn]

LGTM. Thanks for the change!

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-1315-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 56976e1078e445f27f2358cf066ec45a9d012a9a
# Push it to GitHub
git push --set-upstream origin backport/backport-1315-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-1315-to-2.x.