serverutil: Serve metrics on dedicated mux

[1seal]

Rationale

The metrics HTTP listener currently uses http.DefaultServeMux (via http.Handle/http.HandleFunc).
If net/http/pprof is imported elsewhere in the binary (e.g., for debugging), its handlers are
registered on the default mux and become reachable on the metrics endpoint.

While the metrics listener defaults to localhost, deployments may expose it for scraping
(Prometheus / managed monitoring). Using a dedicated mux makes the contract explicit and prevents
unintended handler exposure.

Testing

Added TestHTTPServerDoesNotExposeDefaultServeMux which:

• Imports net/http/pprof (registers on default mux).
• Starts serverutil.Main with an HTTP endpoint.
• Verifies /metrics returns 200.
• Verifies /debug/pprof/ returns 404.

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