refactor(connector): make source connector's metrics guarded

[lmatz]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

fix #17529

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[fuyufjh]

I observed a wired curve online as following

It's probably caused by the usage of Guarded metrics in this PR, which is incorrect.

"Guarded" means that when drop() is called, it removes the label from the Prometheus client.

This PR changes with_label_values to with_guarded_label_values, which results in the metrics being reported once and then immediately dropped.

The correct approach is to place GuardedMetrics in a location where its lifecycle matches that of the object associated with the metrics (for example, placing the metrics of a Source in the SourceExecutor or an internal struct within it). This way, when the SourceExecutor is dropped, the metrics are also dropped simultaneously.