server/resource_group: add allocation observability

[okJiang]

What problem does this PR solve?

Issue Number: ref #10488

Resource control observability on the server side does not clearly show why tokens are granted slowly, whether service_limit is involved, or whether server-side metrics stay correct across cleanup and edge cases.

What is changed and how does it work?

This PR adds server-side allocation metrics and fixes several correctness gaps so the controller and server can be diagnosed together.

server/resource_group: add allocation observability

It includes:

• server metrics for granted tokens, slot counts, slot events, token loan, trickle duration, RU config, and throttling causes
• explicit cause attribution for service_limit versus group_fill_rate_or_burst
• keyspace-name fallback and cleanup fixes so metrics stay continuous
• tests for immediate grant, trickle grant, slot-event accounting, and metric cleanup

Check List

Tests

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

Manual test:

• Verified in a local tiup playground setup with 3 PD, 2 TiDB, and 3 TiKV.
• Ran a shared-service_limit scenario with two resource groups and confirmed Grafana shows service_limit in throttling causes.
• Ran a low-fill-rate scenario and confirmed Grafana shows group_fill_rate_or_burst without false service_limit signals.

Code changes

Side effects

• Increased code complexity

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

Add server-side resource control metrics for allocation state, token trickle, and throttling causes.

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[codecov]

Codecov Report

❌ Patch coverage is 93.23308% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (b21a183) to head (b65de01).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10605      +/-   ##
==========================================
+ Coverage   78.96%   78.98%   +0.01%     
==========================================
  Files         532      532              
  Lines       71883    72083     +200     
==========================================
+ Hits        56766    56937     +171     
- Misses      11093    11116      +23     
- Partials     4024     4030       +6     

Flag	Coverage Δ
unittests	78.98% <93.23%> (+0.01%)	⬆️

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