gRPC server instrumentation creates multiple traces on streaming requests

[alertedsnake]

Environment
Current master code, basically the sample code in the documentation, testing with a unary request vs. a streaming request.

Steps to reproduce
Create a simple gRPC servicer with two RPCs, one which returns a single message (the unary response), and one which yields items in a list for a streaming response.

The key here is to make an instrumented request within the primary request handler (I'm using a simple HTTP get with the Requests instrumentation), so you get an additional span which should be attached to the same trace.

What is the expected behavior?
A single trace with the main span, and a second child span for the HTTP request.

What is the actual behavior?
Two separate traces, each containing a single span.

Additional context
The problem only occurs on streaming requests - I'm sure the reworking I did as part of open-telemetry/opentelemetry-python#1171 is where the problem started, I didn't take into account the streaming case specifically with multiple spans, and naturally, there are no tests for anything streaming, only unary responses.

So as part of this, we'll need some useful tests as well. I'll see if I can write up my test case as an actual test case.

And again, I've got a vested interest in this working, so I'll have a PR up soon.