maruina / go-infrabin / 18802780471

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Add HTTP metrics to Prometheus endpoint (#403)

* Add HTTP metrics to Prometheus endpoint

Implements custom HTTP metrics middleware to track requests going through
the grpc-gateway, addressing issue #48. Previously, only gRPC endpoint
metrics were collected; HTTP layer requests were untracked.

## Changes

- Created `pkg/infrabin/metrics.go` with custom middleware
  - Exposes `infrabin_http_requests_total` counter
  - Exposes `infrabin_http_request_duration_seconds` histogram
  - Implements `normalizeRoute()` to prevent cardinality explosion

- Modified `pkg/infrabin/http.go` to integrate metrics middleware
  - Wraps grpc-gateway handler with `HTTPMetricsMiddleware()`
  - Captures full HTTP request lifecycle including JSON conversions

- Added comprehensive tests
  - `metrics_test.go`: 12 unit tests for route normalization
  - `http_test.go`: Integration test for end-to-end metrics collection

## Design Decisions

- Zero additional dependencies (uses existing prometheus/client_golang)
- Manual route pattern mapping chosen over auto-discovery for simplicity
- Metric namespace: `infrabin_http_*` (distinct from `grpc_server_*`)
- Route normalization prevents high cardinality (e.g., /delay/5 → /delay/{duration})

## Metrics Example

```
infrabin_http_requests_total{handler="/headers",method="GET",status="200"} 15
infrabin_http_request_duration_seconds_sum{handler="/delay/{duration}",method="GET"} 16.234
```

Closes #48

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Co-Authored-By: Claude <noreply@anthropic.com>

* Fix golangci-lint error: use t.Setenv instead of os.Setenv

Replace os.Setenv/os.Unsetenv with t.Setenv() which automatically
handles cleanup and satisfies errcheck linter.

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Co-Authored-By: Claude <noreply@anthropic.com>

* Change HTTP metric handler labels to match gRPC style

Update handler labels to use simple endpoint names (e.g., 'delay', 'headers')
instead o... (continued)

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