chore: add timestamp range to flight meta

[v0y4g3r]

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What's changed and what's your intention?

Add timestamp range to Flight DoPutMetadata to facilitate better ingestion performance.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[Copilot]

Pull request overview

This PR adds timestamp range metadata fields to the Flight protocol's DoPut operation to support better ingestion performance optimization. The changes allow clients to optionally specify time windows for data batches during Flight-based ingestion.

Key changes:

• Adds optional start_timestamp and end_timestamp fields to DoPutMetadata with corresponding getter methods
• Makes several MemtableStats fields public (estimated_bytes, time_range, max_sequence, series_count)

Reviewed changes

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

File	Description
src/common/grpc/src/flight/do_put.rs	Adds optional timestamp range fields to DoPutMetadata for time-windowed batch metadata
src/mito2/src/memtable.rs	Changes visibility of MemtableStats fields from private to public

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[Copilot]

The new timestamp fields (start_timestamp and end_timestamp) have getter methods but no setter methods or builder pattern. This makes the API incomplete - there's no way for clients to populate these fields after construction. Consider adding setter methods (e.g., with_start_timestamp, with_end_timestamp) following the builder pattern used elsewhere in the codebase, or provide a constructor that accepts these optional parameters.

[fengjiachun]

LGTM