Switch away from getMergeInstance for derived source

[jmazanec15]

Description

I was running the DerivedSourceIts for the derived source feature in a multi-node setting with replicas and was noticing that getMergeInstance was being called outside of merge when reindex was happening. I couldnt figure out the stack trace, but decided to revert back to wrapping the readers during merge. This fixed it.

It might be getting called in the SequentialStoredFieldsReader in this hack:

• https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/search/fetch/FetchPhase.java#L154-L157
• https://github.com/opensearch-project/OpenSearch/blob/2847695ad1fcc04c88b96c8bab0bfdf694fa05dc/server/src/main/java/org/opensearch/common/lucene/index/SequentialStoredFieldsLeafReader.java#L71

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

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[shatejas]

Looks good, I don't see unit tests. Please iterate on it in separate PR

I do think the opensearch core shouldn't be using it in a hacky way. It's violating the interface contract. It would be good to raise this issue so the pattern is not repeated at other places.

Approving for now

[opensearch-trigger-bot]

The backport to main 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-main main
# Navigate to the new working tree
cd .worktrees/backport-main
# Create a new branch
git switch --create backport/backport-2472-to-main
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 abae323c6204cfc725e124e0c9487f2d3024a08b
# Push it to GitHub
git push --set-upstream origin backport/backport-2472-to-main
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-main

Then, create a pull request where the base branch is main and the compare/head branch is backport/backport-2472-to-main.