Guard nil metrics before request logging middleware

[haasonsaas]

Summary

• return the request-id wrapper directly when metrics are unavailable
• add a regression covering the nil-metrics handler path

Testing

• go test ./cmd/asb-api -count=1

[cursor]

PR Summary

Low Risk
Low risk: small control-flow change to skip request logging/metrics middleware when metrics is nil, plus a regression test. Main behavior change is that requests still get X-Request-Id but no metrics/logging are recorded in this configuration.

Overview
newObservedHandler now guards against nil metrics by returning only the WithRequestID wrapper, avoiding invoking the request logging/metrics middleware when metrics aren’t configured.

Adds TestNewObservedHandlerAllowsNilMetrics to ensure the handler still serves requests successfully and includes an X-Request-Id header when metrics is nil.

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