Add comment about instanceConfig and fix benchmark driver server name configuration.

(cherry picked from commit b0dcfbfe5c9bdc5628fbf854fc8dade4c526eb0e)

Author: Nikola Grcevski

pinot-controller/src/main/java/org/apache/pinot/controller/helix/core/PinotHelixResourceManager.java

@@ -263,7 +263,7 @@ public ZkHelixPropertyStore<ZNRecord> getPropertyStore() {
    */
   private void addInstanceGroupTagIfNeeded() {
     InstanceConfig instanceConfig = getHelixInstanceConfig(_instanceId);
-    assert instanceConfig != null;
+    // The instanceConfig can be null when connecting as a participant while running from PerfBenchmarkRunner
     if (instanceConfig != null && !instanceConfig.containsTag(Helix.CONTROLLER_INSTANCE)) {
       LOGGER.info("Controller: {} doesn't contain group tag: {}. Adding one.", _instanceId, Helix.CONTROLLER_INSTANCE);
       instanceConfig.addTag(Helix.CONTROLLER_INSTANCE);

pinot-tools/src/main/java/org/apache/pinot/tools/perf/PerfBenchmarkDriver.java

@@ -238,7 +238,7 @@ private void startServer()
     if (_segmentFormatVersion != null) {
       serverConfiguration.setProperty(CommonConstants.Server.CONFIG_OF_SEGMENT_FORMAT_VERSION, _segmentFormatVersion);
     }
-    serverConfiguration.setProperty(CommonConstants.Helix.Instance.INSTANCE_ID_KEY, _serverInstanceName);
+    serverConfiguration.setProperty(CommonConstants.Server.CONFIG_OF_INSTANCE_ID, _serverInstanceName);
     LOGGER.info("Starting server instance: {}", _serverInstanceName);
     new HelixServerStarter(_clusterName, _zkAddress, serverConfiguration);
   }