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). The client should stop sending additional audio
data, half-close the gRPC connection, and wait for any additional results
until the server closes the gRPC connection. This event is only sent if
single_utterance was set to true, and is not used otherwise.