Whitelisting / from Circuit Breaker Exception (#32325) (#32666)

When Circuit Breaker has tripped, certain diagnostic requests like
"_cluster/health" succeed where as request to / fails with
503 Service Unavailable. This behavior is observed because of this
commit f32b700 where certain API paths are whitelisted from
Circuit Breaking exception, but / is not whitelisted.
Added / to circuit breaker whitelist so that it can be used for
diagnostic purposes

server/src/main/java/org/elasticsearch/rest/action/RestMainAction.java

@@ -67,4 +67,9 @@ public class RestMainAction extends BaseRestHandler {
         response.toXContent(builder, request);
         return new BytesRestResponse(RestStatus.OK, builder);
     }
+
+    @Override
+    public boolean canTripCircuitBreaker() {
+        return false;
+    }
 }