ddtrace/tracer/textmap: Added none as a supported propagator for trace context extraction and injection

[dianashevchenko]

What does this PR do?

Following Implementing W3C Trace Context Propagation, which adds none as a supported propagator for trace context extraction and injection.

• When none is the only propagator listed in the inject or extract list, the corresponding trace context operation is disabled.
• If there are other propagators in the inject or extract list, the none propagator has no effect.
• none propagator means no headers will be applied

Motivation

W3C Trace Context Propagation RFC

Describe how to test/QA your changes

• none applies no headers
• none,b3 or any other combination of none with other propagators doesn't change existing functionality and tests

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[ajgajg1134]

Looking at like 197 here it seems like setting a value of none,b3 would result in a message "unrecognized propagator: none". Can we add a special case for none that instead warns the user something like Propagator "none" has no effect when combined with other propagators. To disable propagator set only "none" (Feel free to change the message). (And a quick little unit test update to make sure that future users don't accidentally break that message)

[dianashevchenko]

Good point 👍 Updated

[pr-commenter]

Benchmarks

Comparing candidate commit 87bd5e0 in PR branch shevchenko/none-ppropagator with baseline commit 4b12722 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.