feat: add per-partition convert, result cache metrics

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

This PR adds several metrics to ScanMetricsSet

• add per partition convert cost metrics
• add index result cache hit/miss for each index
• distributor_divider_cost for series divider

Other improvements:

• It fixes an issue that the puffin_metadata bytes were never added.
• It unifies the scan cost between different scanners. Now the scan cost always contains the cost to build parts and readers.
• It removes some unused fields from ScannerMetrics
• It adds flat format flag in the explain verbose content

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.

[waynexia]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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