code: protected void backpatchMessageLength(final int startPosition, final BsonOutput bsonOutput) {
          int messageLength = bsonOutput.getPosition() - startPosition;
          bsonOutput.writeInt32(bsonOutput.getPosition() - messageLength, messageLength);
      }
docs: Backpatches the message length into the beginning of the message.
  
  @param startPosition the start position of the message
  @param bsonOutput the output