Today SignalR invocations do not have any activity associated with them. We currently piggy-back on the default ASP.NET Core activity and it's not great for invocations over the websocket. We should add an activity following the semantic conventions for rpc.

Open questions:

• Should we do this tracing in all directions? Client to server and server to client?
• Do we use message headers to pass this state across the wire seamlessly? This will impact the performance of invocations.

cc @BrennanConroy