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_test.go

@@ -219,3 +219,72 @@ func TestPreflightAllowsOnlyWhatPublicRoutesServe(t *testing.T) {
 		}
 	}
 }
+
+// metricsRequest builds a GET for /metrics from remoteAddr that logs
+// in with the given password.
+func metricsRequest(
+	t *testing.T,
+	remoteAddr string,
+	password string,
+) *http.Request {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/metrics", nil,
+	)
+	req.RemoteAddr = remoteAddr
+	req.SetBasicAuth(metricsUsername, password)
+
+	return req
+}
+
+// TestMetricsRateLimitComesBeforeAuth checks that failed logins to
+// /metrics count against the rate limit; that once an address is over
+// it, even the right password gets 429, with the same body as a wrong
+// one; and that another address still gets in.
+func TestMetricsRateLimitComesBeforeAuth(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_METRICS_USERNAME", metricsUsername)
+	t.Setenv("DNSWATCHER_METRICS_PASSWORD", metricsPassword)
+
+	const (
+		guesser = "198.51.100.1:4000"
+		other   = "198.51.100.2:4000"
+
+		// Far more guesses than the rate limit allows.
+		maxGuesses = 1000
+	)
+
+	srv := routedServer(t)
+
+	var guess *httptest.ResponseRecorder
+
+	for range maxGuesses {
+		guess = serve(srv, metricsRequest(t, guesser, "wrong"))
+		if guess.Code != http.StatusUnauthorized {
+			break
+		}
+	}
+
+	if guess.Code != http.StatusTooManyRequests {
+		t.Fatalf("wrong password: status = %d, want 429", guess.Code)
+	}
+
+	right := serve(srv, metricsRequest(t, guesser, metricsPassword))
+	if right.Code != http.StatusTooManyRequests {
+		t.Errorf("right password: status = %d, want 429", right.Code)
+	}
+
+	if right.Body.String() != guess.Body.String() {
+		t.Errorf(
+			"429 body with right password = %q, with wrong one = %q",
+			right.Body.String(), guess.Body.String(),
+		)
+	}
+
+	rec := serve(srv, metricsRequest(t, other, metricsPassword))
+	if rec.Code != http.StatusOK {
+		t.Errorf("another address: status = %d, want 200", rec.Code)
+	}
+}