adding missing checkpoint and correcting follower stats test case

[sricharanvuppu]

Description

Follower checkpoint stats are missing in TranslogSequencer.
Corrected test Follower stats

Issues Resolved

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.

[monusingh-1]

Follower checkpoint was not being updated based on indexShard.localCheckpoint.
The line that updates followerCheckpoint was missed here 448e7a7#diff-f31917b47c2c6401cae377a4d141dd3c0dd8de19881c405c64fb4e3097f1b65bL95
This was causing follower checkpoint to stay at -1

This also fixes the "follower stats" integration tests, the test tried to start replication from indices which were not created.
Added assertBusy for operations_written as it takes time to replicate all docs.

[codecov]

Codecov Report

Merging #1064 (083c979) into main (96299ef) will decrease coverage by 27.05%.
The diff coverage is 100.00%.

❗ Current head 083c979 differs from pull request most recent head bccdf57. Consider uploading reports for the commit bccdf57 to get more accurate results

@@              Coverage Diff              @@
##               main    #1064       +/-   ##
=============================================
- Coverage     73.17%   46.13%   -27.05%     
+ Complexity     1021      633      -388     
=============================================
  Files           141      141               
  Lines          4768     4769        +1     
  Branches        540      540               
=============================================
- Hits           3489     2200     -1289     
- Misses          950     2305     +1355     
+ Partials        329      264       -65     

Impacted Files	Coverage Δ
...search/replication/task/shard/TranslogSequencer.kt	67.69% <100.00%> (-5.75%)	⬇️

... and 74 files with indirect coverage changes

[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-1064-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 324b10337ca46c57579186b21f74c9f9d9fd2889
# Push it to GitHub
git push --set-upstream origin backport/backport-1064-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-1064-to-2.x.

[opensearch-trigger-bot]

The backport to 2.9 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-2.9 2.9
# Navigate to the new working tree
cd .worktrees/backport-2.9
# Create a new branch
git switch --create backport/backport-1064-to-2.9
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 324b10337ca46c57579186b21f74c9f9d9fd2889
# Push it to GitHub
git push --set-upstream origin backport/backport-1064-to-2.9
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.9

Then, create a pull request where the base branch is 2.9 and the compare/head branch is backport/backport-1064-to-2.9.