Improve entity stats when hovered

[emilk]

What

When hovering an entity in e.g. the timeline view or blueprint view, we used to show the amount of memory used by that entity, as well as a data rate. It now also includes also the number of log rows.

To do this I improved the stats pipeline of re_chunk_store and EntityDb.

Best reviewed commit-by-commit

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

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[teh-cmc]

At this point we might as well move these two to stats.rs I guess