Since there is a new IDL based type for `TelemetryContext`, we can not add this type as a stack based attribute on `OpMsg` (or even include the header for `TelemetryContext`) because all IDL types include `op_msg.h`.  If we did the compiler would be trapped in a circular reference.  

For now `OpMsg` uses an incomplete type to `TelemetryContext` and heap allocates the instance.  Once the circular reference is broken, we should move `TelemetryContext` to be an optional instead.