@Override
  void poll(RequestRegistrationCallback<GetOperation> requestRegistrationCallback) {
    //First, check if any of the existing requests have timed out.
    cleanupExpiredInFlightRequests(requestRegistrationCallback);
    checkAndMaybeComplete();
    if (!isOperationComplete()) {
      fetchRequests(requestRegistrationCallback);
    }
  }
For this operation, create and populate get requests (in the form of {@link RequestInfo}) to send out.
@param requestRegistrationCallback the {@link RequestRegistrationCallback} to call for every request that gets
                                   created as part of this poll operation.
ambry-router/src/main/java/com/github/ambry/router/GetBlobInfoOperation.java