Add error content on metric counter with attributes

[joaopaulovieira]

This PR improves error metric recording and testing in the httpclient module.

request.go

• Bug fix:
  • Ensures the attrs map is always initialized before adding the "error" field in error cases, preventing panic (assignment to entry in nil map).
• Relevant change:
  • Key line changed:
    • Before: var attrs map[string]string
    • After: attrs := map[string]string{}
  • Always adds the error field to attrs when a generic error occurs.

register_metrics_test.go

• New tests added:
  • Tests for error scenarios:
    • Circuit breaker open error (ErrCircuitOpen): validates increment of the circuit_open counter.
    • Generic error: validates increment of the errors counter and the registration of the error attribute.
• Improved assertions:
  • Tests now validate not only the existence of counters, but also the suffixes of the keys and the registered attributes.
  • Uses WithChainCallback to simulate errors via a custom callback.

How to test

• Run go test on register_metrics_test.go to ensure all success and error scenarios are covered and there are no more panics.