expose hedge stats via Prometheus-style /metrics handler

[batazor]

Close #2

[Copilot]

Pull request overview

Adds a Prometheus-compatible metrics exposition path for hedge.Stats, enabling consumers to scrape hedge transport counters via an HTTP /metrics endpoint.

Changes:

• Extend Stats with WritePrometheus(io.Writer) and ServeHTTP(http.ResponseWriter, *http.Request) for Prometheus text exposition.
• Make Snapshot() / HedgeRate() nil-safe.
• Add HTTP handler tests and document /metrics usage in the README.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
stats.go	Implements Prometheus text exposition output and an HTTP handler on Stats.
hedge_test.go	Adds tests covering /metrics GET and method-not-allowed behavior.
README.md	Documents how to mount stats as a Prometheus-style /metrics endpoint.

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