metrics: rate limit /metrics per client address before Basic Auth (closes #101)

/metrics is behind a password, and REPO_POLICIES.md requires rate
limiting on password logins. Each client address may now send it 30
requests a minute, counted by httprate before Basic Auth, so failed
logins use up the allowance and a request over it gets 429 without
the password being checked. The address is the one the existing
trusted-proxy logic in internal/middleware works out, with IPv6
addresses grouped by /64; an IPv4 address a proxy reports in
IPv6-mapped form counts as the plain IPv4 address. A Prometheus
server scraping every 15 seconds sends 4 requests a minute.

Model: opus-5-5

internal/server/routes.go

@@ -64,9 +64,12 @@ func (s *Server) SetupRoutes() {
 	// Prometheus scraper is not a browser. It is mounted rather than
 	// added with Get so that every method on /metrics, OPTIONS
 	// included, ends here instead of falling through to the public
-	// router and its CORS.
+	// router and its CORS. The rate limit comes before Basic Auth, so
+	// failed logins count against it and a request over the limit
+	// never reaches the password check.
 	if s.params.Config.MetricsUsername != "" {
 		metrics := chi.NewRouter()
+		metrics.Use(s.mw.MetricsRateLimit())
 		metrics.Use(s.mw.MetricsAuth())
 		metrics.Get("/", promhttp.Handler().ServeHTTP)
 		s.router.Mount("/metrics", metrics)