refactor(dashboard): refactor back-pressure calculation

[fuyufjh]

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What's changed and what's your intention?

Based on the previous PR, this PR rewrite the back-pressure calculation code in much less code.

I did this because I am going to add more statistics to the Web dashboard, such as streaming throughput.

Due to my mistake, #19954 was incorrectly merged into graphite-base/19954

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