Add activities to SignalR invocations

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](https://opentelemetry.io/docs/specs/semconv/rpc/rpc-spans/).

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