This event indicates that the server has detected the end of the user's
speech utterance and expects no additional speech. Therefore, the server
will not process additional audio (although it may subsequently return
additional results). When the client receives 'END_OF_SINGLE_UTTERANCE'
event, the client should stop sending the requests. However, clients
should keep receiving remaining responses until the stream is terminated.
To construct the complete sentence in a streaming way, one should
override (if 'is_final' of previous response is false), or append (if
'is_final' of previous response is true). This event is only sent if
single_utterance was set to true, and is not used otherwise.
