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. A Prometheus server scraping every 15
seconds sends 4 requests a minute.

Model: opus-5-5

internal/middleware/middleware.go

@@ -11,6 +11,7 @@ import (
 	"github.com/99designs/basicauth-go"
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/go-chi/cors"
+	"github.com/go-chi/httprate"
 	"go.uber.org/fx"
 
 	"sneak.berlin/go/dnswatcher/internal/config"
@@ -21,6 +22,17 @@ import (
 // corsMaxAge is the maximum age for CORS preflight responses.
 const corsMaxAge = 300
 
+// Rate limit for /metrics: each client address may send
+// metricsRequestLimit requests per metricsRequestWindow. Every request
+// counts, so password guessing gets at most 30 tries a minute per
+// address. One Prometheus server scraping every 15 seconds sends 4
+// requests a minute, and two scraping every 5 seconds from one address
+// send 24, so normal scraping stays under the limit.
+const (
+	metricsRequestLimit  = 30
+	metricsRequestWindow = time.Minute
+)
+
 // Security response header values applied to every response.
 //
 // The CSP is as strict as the dashboard allows: the template ships no
@@ -268,6 +280,23 @@ func (m *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
 	}
 }
 
+// MetricsRateLimit returns middleware for /metrics that answers 429
+// Too Many Requests to a client address over the rate limit. The
+// address is the one realIP works out, so a client that is not a
+// trusted proxy cannot get a fresh allowance by sending its own
+// X-Real-IP or X-Forwarded-For. CanonicalizeIP counts all IPv6
+// addresses in one /64 as one client, since a client usually holds a
+// whole /64.
+func (m *Middleware) MetricsRateLimit() func(http.Handler) http.Handler {
+	return httprate.LimitBy(
+		metricsRequestLimit,
+		metricsRequestWindow,
+		func(request *http.Request) (string, error) {
+			return httprate.CanonicalizeIP(realIP(request)), nil
+		},
+	)
+}
+
 // MetricsAuth returns basic auth middleware for /metrics.
 func (m *Middleware) MetricsAuth() func(http.Handler) http.Handler {
 	if m.params.Config.MetricsUsername == "" {