Feature request: Support traceparent header

[goodspark]

This is similar to this feature request made on the JS library: https://github.com/DataDog/dd-trace-js/issues/1789

https://www.w3.org/TR/trace-context/#examples-of-http-traceparent-headers

We're using a self-hosted 3rd party service that uses OpenTelemetry for tracing and sending it to an OpenTelemetry ingest endpoint on Datadog agents. The service uses traceparent headers for outgoing HTTP requests to support distributed tracing, but I don't think the Python library supports this (at least, the term doesn't show up from a source search).

As referenced in the JS library issue, here's the spec on traceparent: https://www.w3.org/TR/trace-context/#traceparent-header