gRPC client interceptor: incorrect parent span attribution for spans following asynchronous calls

[evantorrie]

Describe the bug

Spans created after the completion of a gRPC client call are parented to the gRPC call span rather than the span that was active at the beginning of the client call.

Steps to reproduce
Create a span (P). Make an asynchronous unary gRPC call (A) using ClientCalls.futureUnaryCall. Set up a callback on completion to invoke another gRPC client call (B).

What did you expect to see?

Spans A and B should have the same parent (P)
Span A should end before Span B begins.

What did you see instead?

Span B is a child of Span A.
Span A continues until the completion of span B.

What version are you using?
v1.5.0

Environment
JDK: AdoptOpenJDK8
OS: RHEL7

Additional context
Discussed at Java instrumentation office-hours with @anuraaga, @trask and @jkwatson