feat(dashboard): show current epoch and color for fragments & relations

[fuyufjh]

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What's changed and what's your intention?

Show current epoch for fragments & relations in dashboard, and color the nodes into yellow or red color when the epoch fall behind NOW too much (see latencyToColor()).

Also, added a mouse-hovering tooltip to show more information about a fragment or relation node.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

• My PR needs documentation updates.

Release note

[fuyufjh]

Hmm, Prometheus data source is not done yet. Let me fire another PR.