Test metrics auth, CORS preflight, login logging and metrics (closes #79)

New tests only. MetricsAuth on its own answers 401 with a challenge
without credentials or with a wrong username or password, and lets the
configured ones through. A CORS preflight request gets the same
Access-Control-Allow-Origin as a GET. A POST / carrying the signing key
leaves the key out of the request log line, and the login handler's own
log lines leave out the submitted key. The metrics middleware on its own
records a request it served; the router records nothing while no metrics
username is set.

Not tested through the router: the basic auth in front of /metrics and
recording with a metrics username set (#180).

The pinned basicauth-go compares the password in constant time.

Model: opus-5-5

internal/server/metrics_internal_test.go

@@ -0,0 +1,32 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// TestNoMetricsRecordedWithoutMetricsUsername checks that with no metrics
+// username set the router records nothing about the requests it serves.
+// /metrics is not served then, so the process-wide Prometheus registry is
+// read directly. TestMaintenanceModeKeepsOtherRoutes records into the same
+// registry, but never a GET /robots.txt.
+func TestNoMetricsRecordedWithoutMetricsUsername(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+	s.ServeHTTP(httptest.NewRecorder(), httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/robots.txt", nil))
+
+	rec := httptest.NewRecorder()
+	promhttp.Handler().ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/metrics", nil))
+
+	if strings.Contains(rec.Body.String(), `handler="/robots.txt"`) {
+		t.Errorf("with no metrics username GET /robots.txt was recorded:\n%s",
+			rec.Body.String())
+	}
+}