code: public ListInstancesResponse listInstances(ListInstancesRequest request) {
          checkNotNull(request, "request should not be null.");
          checkStringNotEmpty(request.getClusterId(), "The parameter clusterId should not be null or empty string.");
          checkStringNotEmpty(request.getInstanceGroupId(),
                  "The parameter instanceGroupId should not be null or empty string.");
  
          InternalRequest internalRequest = this.createRequest(
                  request, HttpMethodName.GET, CLUSTER, request.getClusterId(), INSTANCE_GROUP,
                  request.getInstanceGroupId(), INSTANCE);
  
          return this.invokeHttpClient(internalRequest, ListInstancesResponse.class);
      }
docs: List the instances belonging to the target instance group in the BMR cluster.
  
  @param request containing the ID of target BMR cluster and the ID of the instance group.
  @return The response containing a list of Instance objects.