Event indicates that the server has detected the end of the user's speech
utterance and expects no additional inputs.
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 message is only sent if single_utterance was
set to true, and is not used otherwise.