Value of the HTTP User-Agent header sent by the client.
@example CERN-LineMode/2.15 libwww/2.17b3
@example Mozilla/5.0 (iPhone; CPU iPhone OS 14_7_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.1.2 Mobile/15E148 Safari/604.1
@example YourApp/1.0.0 grpc-java-okhttp/1.27.2