Trace Tags Propagation in Distributed Traces

[marcotc]

This implements the propagation of Datadog-specific trace tags across a distributed trace's lifecycle.

Trace tags starting with the string _dd.p. are considered Datadog distributed tags, are are propagated through the new x-datadog-tags header.

The x-datadog-tags header is consumed from upstream calls, if present, and inject into downstream calls when present.

The first value for a distributed tag is _dd.p.dm: Sampling Decision Maker. This tag represents what mechanism was used to make a sampling decision.

When the sampling decision comes from an upstream context, ddtrace propagates that value downstream, as well as setting it as a trace-level tag. This tag will be flushed with trace payload.

When the sampling decision is made in the instrumented Ruby application, the value of _dd.p.dm is populated locally and propagated downstream.

[codecov-commenter]

Codecov Report

Merging #2260 (f58ccad) into master (f4791d1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           master    #2260    +/-   ##
========================================
  Coverage   98.33%   98.33%            
========================================
  Files        1100     1102     +2     
  Lines       58334    58613   +279     
========================================
+ Hits        57361    57638   +277     
- Misses        973      975     +2     

Impacted Files	Coverage Δ
lib/datadog/tracing/propagation/http.rb	100.00% <ø> (ø)
lib/datadog/core/configuration/settings.rb	97.47% <100.00%> (+0.03%)	⬆️
lib/datadog/tracing/configuration/ext.rb	100.00% <100.00%> (ø)
.../datadog/tracing/distributed/datadog_tags_codec.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/distributed/headers/datadog.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/distributed/headers/ext.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/metadata/ext.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/trace_digest.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/trace_operation.rb	98.87% <100.00%> (+0.01%)	⬆️
lib/datadog/tracing/tracer.rb	95.29% <100.00%> (+0.02%)	⬆️
... and 11 more

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[delner]

Pretty big, but good to see this ready to go! Thanks for your hard work, and patience with reviews on this one.

If you think it would be helpful, I would encourage sanity-testing a pre-release build in some kind of controlled environment. But that shouldn't block a merge at this point.