w3c(phase 3): ensure last datadog parent id is always recorded

[mabdinur]

What does this PR do?

Ensures when trace headers with the same trace_id but different span_ids are received by an application the tracecontext headers are treated as the source of truth.

• The traceparent header will be used to set the parent_id
• If the conflicting trace information was extracted from datadog headers, the span_id from the datadog headers will be stored in the _dd.parent_id parent_id tag.
• All other propagation styles are third class citizens. If they conflict with tracecontext headers their span_ids will be overwritten.

This behavior will be validated by the following system-tests: DataDog/system-tests#2385

Motivation:

Provide first class support for services instrumented with opentelemetry and reduce instances of "broken traces".

Additional Notes:

How to test the change?

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.96970% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 98.13%. Comparing base (bbef35a) to head (3227a29).
Report is 102 commits behind head on master.

Files	Patch %	Lines
lib/datadog/tracing/distributed/propagation.rb	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3631      +/-   ##
==========================================
- Coverage   98.13%   98.13%   -0.01%     
==========================================
  Files        1223     1223              
  Lines       72115    72185      +70     
  Branches     3424     3435      +11     
==========================================
+ Hits        70768    70836      +68     
- Misses       1347     1349       +2     

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