feat: support w3c trace context

[ZStriker19]

Description

We want to support W3C propagation headers which is what Otel uses. w3c standard.
Currently we're only adding support for traceparent to propagate the trace context. We are not yet adding support for tracestate but are planning to in future work.

Motivation

We want to support otel users and possibly other future tracers that use this format for distributed tracing

Design

Overview:

  - ``traceparent`` describes the position of the incoming request in its
    trace graph in a portable, fixed-length format. Its design focuses on
    fast parsing. Every tracing tool MUST properly set traceparent even when
    it only relies on vendor-specific information in tracestate

The format for ``traceparent`` is::

  HEXDIGLC        = DIGIT / "a" / "b" / "c" / "d" / "e" / "f"
  value           = version "-" version-format
  version         = 2HEXDIGLC
  version-format  = trace-id "-" parent-id "-" trace-flags
  trace-id        = 32HEXDIGLC
  parent-id       = 16HEXDIGLC
  trace-flags     = 2HEXDIGLC

Example value of HTTP ``traceparent`` header::

    value = 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01
    base16(version) = 00
    base16(trace-id) = 4bf92f3577b34da6a3ce929d0e0e4736
    base16(parent-id) = 00f067aa0ba902b7
    base16(trace-flags) = 01  // sampled

Implementation details:

  - Datadog Trace and Span IDs are 64-bit unsigned integers.
  - The W3C Trace Context Trace ID is a 32-byte hexadecimal string. This is
    transformed for propagation between Datadog by taking the lower
    16-bytes.

Testing strategy

I've added tests similar to those for b3, making sure that we're converting the W3C header correctly into a context via _extract() and that we're converting the context correctly into the traceparent W3C header via `_inject().

Checklist

• 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.

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.

[Kyle-Verhoog]

A few things, but overall good with the approach and testing 👍

[codecov-commenter]

Codecov Report

Merging #4170 (11dddbd) into 1.x (e315198) will increase coverage by 0.00%.
The diff coverage is 89.83%.

@@           Coverage Diff           @@
##              1.x    #4170   +/-   ##
=======================================
  Coverage   78.08%   78.09%           
=======================================
  Files         734      734           
  Lines       58874    58923   +49     
=======================================
+ Hits        45974    46016   +42     
- Misses      12900    12907    +7     

Impacted Files	Coverage Δ
ddtrace/propagation/http.py	94.75% <88.46%> (-1.56%)	⬇️
ddtrace/internal/constants.py	100.00% <100.00%> (ø)
tests/tracer/test_propagation.py	100.00% <100.00%> (ø)
ddtrace/internal/telemetry/writer.py	76.82% <0.00%> (-0.67%)	⬇️

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[Kyle-Verhoog]

just one thought, otherwise lgtm!

[majorgreys]

Doc changes otherwise lgtm

[ZStriker19]

@mabdinur