metrics: Add Backfill task type and update metric query format

[okJiang]

What problem does this PR solve?

Issue Number: close #49621

Problem Summary:

What changed and how does it work?

update metric query format

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

has been fixed in https://github.com/pingcap/tidb/pull/50071/files#diff-49c216fe673e1484dc2b812ffd4da6ebbbc40673598fae7a0ae3f334d87c09c2R33
backfill prefix become normal after 17:25, because of the fix

optimization
before

after

• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[codecov]

Codecov Report

Merging #50249 (f695c63) into master (528a6e3) will decrease coverage by 16.4074%.
Report is 3 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #50249         +/-   ##
=================================================
- Coverage   71.8613%   55.4540%   -16.4074%     
=================================================
  Files          1442       1554        +112     
  Lines        346790     587032     +240242     
=================================================
+ Hits         249208     325533      +76325     
- Misses        77254     238798     +161544     
- Partials      20328      22701       +2373     

Flag	Coverage Δ
integration	36.6834% <ø> (?)
unit	70.0749% <ø> (-1.7865%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	54.0269% <ø> (-2.2860%)	⬇️
parser	∅ <ø> (∅)
br	55.5618% <ø> (+4.2527%)	⬆️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, ywqzzy

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-10 03:51:31.686342097 +0000 UTC m=+415281.270595780: ☑️ agreed by ywqzzy.
• 2024-01-10 04:40:57.150070024 +0000 UTC m=+418246.734323715: ☑️ agreed by lance6716.