code: public void putInFlightMessage(int streamId, OutboundMsgHolder inFlightMessage) {
          if (LOG.isDebugEnabled()) {
              LOG.debug("In flight message added to channel: {} with stream id: {}  ", this, streamId);
          }
          inFlightMessages.put(streamId, inFlightMessage);
      }
docs: Adds a in-flight message.
  
  @param streamId        stream id
  @param inFlightMessage {@link OutboundMsgHolder} which holds the in-flight message