Applications can customize the user-agent header that will be used with their RPCs (by means of channel arguments in the core-wrapping languages), but we offer no guidance on what applications should do. grpc.primary_user_agent and grpc.secondary_user_agent are different things, and each can be specified multiple times in a single grpc_channel_args value; what does that all mean?

Also it was a complete surprise to me that if an application tries to set the user-agent as though it were an ordinary piece of metadata, it gets eaten by the core and not sent on the wire - we should make that clear too.