@WithSpan and customTextMapPropagator

[mnadeem]

Describe the bug

This is related to #2436

@WithSpan could be more robust if there is a way to specify custom Propagator to extract parent trace.

Even though there is parent trace a new trace is created.

This limits the use of @WithSpan, if a library is not instrumented. And we would be forced to create span the hard way.

Steps to reproduce
use this project : https://github.com/mnadeem/boot-opentelemetry-tempo

What did you expect to see?

A easy and custom way for @WithSpan to extract parent trace, if a library is not instrumented.

What did you see instead?
A clear and concise description of what you saw instead.

What version are you using?
(e.g., v0.9.0, 393e4a2, etc)

Environment
Compiler: (e.g., "AdoptOpenJDK 11.0.6")
OS: (e.g., "Ubuntu 20.04")
Runtime (if different from JDK above): (e.g., "Oracle JRE 8u251")
OS (if different from OS compiled on): (e.g., "Windows Server 2019")

Additional context
Add any other context about the problem here.

[trask]

cc @zeitlinger @jack-berg related to SIG discussion about open-telemetry/opentelemetry-java-contrib#759 and a convenience API for interacting with propagators