Fail loudly on half-set metrics auth credentials (closes #205) (#216)

internal/server/routes.go

@@ -55,8 +55,11 @@ func (s *Server) setupGlobalMiddleware() {
 	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())
 
-	// Metrics middleware (only if credentials are configured)
-	if s.params.Config.MetricsUsername != "" {
+	// Metrics recording middleware, registered only when the
+	// endpoint that exposes what it records is served. The
+	// condition is the same MetricsAuthEnabled the /metrics mount
+	// in setupRoutes reads.
+	if s.params.Config.MetricsAuthEnabled() {
 		s.router.Use(s.mw.Metrics())
 	}
 
@@ -103,8 +106,14 @@ func (s *Server) setupRoutes() {
 		s.h.HandleHealthCheck(),
 	)
 
-	// set up authenticated /metrics route:
-	if s.params.Config.MetricsUsername != "" {
+	// Authenticated /metrics route. The condition is
+	// Config.MetricsAuthEnabled and never the username alone: a
+	// username with an empty password would otherwise mount the
+	// endpoint behind a credential map that accepts an empty
+	// password. Config rejects that combination at startup, and
+	// this reads the same value the startup log reports, so the
+	// two cannot disagree about whether the route exists.
+	if s.params.Config.MetricsAuthEnabled() {
 		s.router.Group(func(r chi.Router) {
 			r.Use(s.mw.MetricsAuth())
 			r.Get(