fix(rpc middleware): fix `is_error` bug (#11951)

* fix(rpc middleware): fix `is_error` bug

* Update client/rpc-servers/src/middleware.rs

substrate/client/rpc-servers/src/middleware.rs

@@ -176,7 +176,9 @@ impl Middleware for RpcMiddleware {
 			.with_label_values(&[
 				self.transport_label,
 				name,
-				if success { "true" } else { "false" },
+				// the label "is_error", so `success` should be regarded as false
+				// and vice-versa to be registrered correctly.
+				if success { "false" } else { "true" },
 			])
 			.inc();
 	}