Add http debug listener

[mcpherrinm]

This adds a debugAddr config, with the address to listen for metrics and pprof. This code is modelled after Boulder's newStatsRegistry.

This just registers basic go stats as well as the promhttp stats. We want to add more stats in the future, but this seemed like a good starting point.

The integration test just check that metrics is reachable and has an expected substring.

Part of #51

[Copilot]

Pull request overview

Adds an optional debug HTTP listener to expose Prometheus metrics and pprof endpoints, and wires a shared Prometheus registry into the main HTTPS handler to record HTTP request metrics.

Changes:

• Introduces stats.New to create a Prometheus registry and (optionally) start a debug HTTP server exposing /metrics and /debug/pprof/*.
• Threads a prometheus.Registerer through main -> server.Run -> newHandler and instruments the main handler with an HTTP requests counter.
• Extends config/test and integration assets to include debugAddr, and adds an integration check for /metrics.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
stats/stats.go	New stats/debug server with Prometheus registry + pprof and /metrics endpoints.
server/server.go	Passes Prometheus registry into handler construction.
server/page.go	Instruments the main HTTPS handler with a Prometheus HTTP requests counter.
server/page_test.go	Updates handler types/calls for the new newHandler signature.
main.go	Creates the registry/debug server and passes registry into server.Run.
config/config.go	Adds DebugAddr to configuration schema.
config/config_test.go	Updates expected config to include DebugAddr.
config/testdata/test.json	Adds debugAddr to test config fixture.
integration/test-certs-site-config.json	Adds debugAddr for integration environment.
integration/integration_test.go	Adds a /metrics reachability check.
docker-compose.yml	Exposes debug port 9001 for local integration.
go.mod / go.sum	Adds Prometheus client deps and updates indirect deps.

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