update docs

Signed-off-by: Ben Ye <[email protected]>

Author: yeya24

docs/configuration/config-file-reference.md

@@ -5992,6 +5992,11 @@ grpc_tls_config:
 # CLI flag: -server.grpc.keepalive.ping-without-stream-allowed
 [grpc_server_ping_without_stream_allowed: <boolean> | default = true]
 
+# Enable Channelz for gRPC server. A web UI will be also exposed on the HTTP
+# server at /channelz
+# CLI flag: -server.enable-channelz
+[enable_channelz: <boolean> | default = false]
+
 # Output log messages in the given format. Valid formats: [logfmt, json]
 # CLI flag: -log.format
 [log_format: <string> | default = "logfmt"]