dd-trace-py feat: Support w3 traceparent headers

Fixes #3872

Description

Supports distributed tracing for services that want to use the W3 trace propagation headers. https://www.w3.org/TR/trace-context

Checklist

[x] Title must conform to conventional commit.
[ ] Add additional sections for feat and fix pull requests.
[ ] Ensure tests are passing for affected code.
[ ] Library documentation and/or Datadog's documentation site is updated. Link to doc PR in description.

Motivation

Design

Testing strategy

Reviewer Checklist

[ ] Title is accurate.
[ ] Description motivates each change.
[ ] No unnecessary changes were introduced in this PR.
[ ] PR cannot be broken up into smaller PRs.
[ ] Avoid breaking API changes unless absolutely necessary.
[ ] Tests provided or description of manual testing performed is included in the code or PR.
[ ] Release note has been added for fixes and features, or else changelog/no-changelog label added.
[ ] All relevant GitHub issues are correctly linked.
[ ] Backports are identified and tagged with Mergifyio.
[ ] Add to milestone.

Jun 28 '22 06:06 goodspark

Hey @goodspark thanks for opening this PR!

We have been discussing lately about adding support for w3c context propagation headers. We are going to be discussing this internally over the next 1-2 weeks to solidify some of the details on how we want to support it (how to enable, what are the settings called, etc).

This is an awesome start to that work, but I want to wait until we have a more clearly defined specification for how we want this implemented across all languages so we don't have to come back to it later.