o11y: Add request error counters

[tao12345666333]

What type of PR is this?

o11y: Add request error counters

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #122

Release Notes: Yes/No

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 src

Owners: @rootfs, @Xunzhuo, @wangchen615
Files changed:

• src/semantic-router/pkg/extproc/error_metrics_test.go
• src/semantic-router/pkg/extproc/processor.go
• src/semantic-router/pkg/extproc/request_handler.go
• src/semantic-router/pkg/extproc/response_handler.go
• src/semantic-router/pkg/metrics/metrics.go
• src/semantic-router/pkg/utils/http/response.go

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🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.

[rootfs]

can you add to the doc too in a followup PR? thanks

[tao12345666333]

Sure! #133