[DOC][BUG] Default Index Merge Policy is Not Customizable for Cluster Level Settings

[IanHoang]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
The current documentation informs users that they can switch OpenSearch merge policies by overriding the cluster-level settings. However, after confirming with @jainankitk, @rishabhmaurya, and @ansjcy merge policies can only be changed on an index-level basis.

The following section of the documentation is misleading and should be removed. When users follow the documentation and attempt to update the default_index_merge_policy, they encounter a validation error:

indices.time_series_index.default_index_merge_policy (String): This setting allows you to specify the default merge policy for time-series indexes, particularly for those with an @timestamp field, such as data streams. The two available options are tiered (default) and log_byte_size. Using log_byte_size for time-series indexes is recommended for enhancing the performance of range queries with the @timestamp field. To override the merge policy on a per-index basis, you can use the index.merge.policy index setting.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
How to recreate error:

1. Setup an opensource OpenSearch cluster. Ensure that you can curl against it.
2. Run this command against your cluster endpoint. A validation error is encountered.

curl -XPUT '<cluster_endpoint>/_cluster/settings' -H 'Content-Type: application/json' -d '{
  "indices" : {
    "time_series_index.default_index_merge_policy": "log_byte_size"
  }
}'
{"error":{"root_cause":[{"type":"action_request_validation_exception","reason":"Validation Failed: 1: no settings to update;"}],"type":"action_request_validation_exception","reason":"Validation Failed: 1: no settings to update;"},"status":400}

[jainankitk]

Thanks @IanHoang for filing this issue. We should move the cluster setting to static section from dynamic.

[hdhalter]

Thanks for filing the issue, @IanHoang. Do you want to remove the cluster setting or move it? We don't have a section for static cluster-level settings, but can create one. If easier to do than explain, please update here: https://github.com/opensearch-project/documentation-website/edit/main/_install-and-configure/configuring-opensearch/index-settings.md. Thanks!

[IanHoang]

Synced up with Heather and we can just remove it since it's already in the static section