Add Remote Store Metadata API Reference Documentation

[dev-docs-github-app-dev]

This PR adds comprehensive API reference documentation for the Remote Store Metadata API, which enables users to fetch metadata about remote store segments and translogs for specific indices and shards.

Key additions include:

• Detailed endpoint descriptions for /_remotestore/metadata/{index} and /_remotestore/metadata/{index}/{shard_id}
• Complete parameter documentation for both path and query parameters
• Clear example requests and responses with detailed JSON structure
• Comprehensive field descriptions for response body components
• Information about required permissions and security considerations
• Usage notes and best practices

This documentation will help users understand and effectively utilize the Remote Store Metadata API for enhanced observability and debugging of indices backed by remote storage.

File added:

• opensearch/API_REFERENCE.md

[dev-docs-github-app-dev]

This pull request was created by AI Agent. Please review the changes and provide feedback. Context used:

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