Correct username in vendor-agnostic metrics guide

apps/docs/content/guides/observability/metrics/vendor-agnostic.mdx

@@ -24,7 +24,7 @@ No matter which collector you use, you need to hit the Metrics API once per minu
   metrics_path: /customer/v1/privileged/metrics
   scheme: https
   basic_auth:
-    username: username
+    username: service_role
     password: '<secret API key (sb_secret_...)>'
   static_configs:
     - targets: