fix: handle TiKV metrics fetch errors (#6897)

[ti-chi-bot]

This is an automated cherry-pick of #6897

What

GetLeaderCount could say the leader metric was missing when the /metrics fetch itself failed. The fetch runs in a goroutine, but the error was stored in a shared var, so the caller could miss it. Tiny fix, but yeah, kinda sus.

This waits for the fetch result through a buffered error channel before returning the fallback "metric not found" error.

Related: #4281 is the same code path, but a different old goroutine-leak issue.

Repro

The added test reproduces it on the old code:

go test ./pkg/tikvapi/v1 -run TestTiKVClient_GetLeaderCountFetchError -count=1

It serves /metrics with HTTP 500. Old code returns "metric not found". New code returns the real fetch error. This can happen IRL when the TiKV metrics endpoint is unavailable, times out, hits TLS/transport errors, or returns non-200; no weird cloud quota edge case needed.

Tests

go test -race ./pkg/tikvapi/v1 -count=1
go test ./pkg/tikvapi/v1 -run TestTiKVClient_GetLeaderCountFetchError -count=100
go test ./pkg/pdapi/... ./pkg/tikvapi/... ./pkg/resourcemanagerapi/... ./pkg/ticdcapi/...
git diff --check
make unit

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.63%. Comparing base (e5ddd8b) to head (5788e5c).

Additional details and impacted files

@@               Coverage Diff               @@
##           release-2.1    #6898      +/-   ##
===============================================
+ Coverage        37.61%   37.63%   +0.01%     
===============================================
  Files              392      392              
  Lines            22483    22485       +2     
===============================================
+ Hits              8458     8463       +5     
+ Misses           14025    14022       -3     

Flag	Coverage Δ
unittest	37.63% <100.00%> (+0.01%)	⬆️

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