[metrics] Add KV snapshot upload throughput metrics

[platinumhamburg]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Fluss exposes remoteLogCopyBytesPerSecond to measure remote log upload throughput, but there is no equivalent metric for KV snapshot uploads.

KV snapshots upload newly created SST files and other snapshot files to remote storage. Without upload throughput metrics, users cannot observe the remote write traffic generated by KV snapshots or assess its contribution to the overall remote storage write workload.

Solution

Expose the following metrics in the existing table-level KV metric group:

Metric	Type	Description
remoteKvCopyBytes	Counter	Cumulative bytes of successfully uploaded KV snapshot files.
remoteKvCopyBytesPerSecond	Meter	KV snapshot upload throughput in bytes per second, derived from the same counter.

Record the uploaded file size in KvSnapshotDataUploader after each file upload completes successfully.

The metrics should follow these semantics:

• Include newly uploaded SST files and other snapshot files handled by the uploader.
• Exclude SST files reused from previous snapshots without uploading.
• Count successful file uploads even if another file upload or the subsequent snapshot commit fails.
• Count successful reuploads again, since they generate additional write traffic.
• Support concurrent file uploads with a thread-safe counter.

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!

[chaohaochen]

I'm willing to submit a PR!