Propagation:Ensure W3C tracestate is always propagated

[marcotc]

What does this PR do?

This PR ensures that the tracestate header received, even if not part of the first valid propagation style, is still propagated downstream. This respects the spec for tracestate, which states:

Vendors receiving a tracestate request header MUST send it to outgoing requests.

This is only an issue when W3C extraction style is proceeded by another style. And this is currently the default (Datadog,tracecontext).
Now with this change, when Datadog headers match, we will now continue searching for tracecontext headers to enrich the downstream propagation context.

Additional Notes:

This PR also introduces the DD_TRACE_PROPAGATION_EXTRACT_FIRST configuration, to allow users to restore the prior behavior of not inspecting any headers after the first one was extracted.

How to test the change?

All changes are covered by unit tests and system-tests (DataDog/system-tests#1795).

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[aliciascott]

Looks good

[codecov-commenter]

Codecov Report

Merging #3255 (8b6432a) into master (8fe9497) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3255   +/-   ##
=======================================
  Coverage   98.21%   98.22%           
=======================================
  Files        1253     1253           
  Lines       72110    72198   +88     
  Branches     3346     3350    +4     
=======================================
+ Hits        70823    70916   +93     
+ Misses       1287     1282    -5     

Files	Coverage Δ
lib/datadog/tracing/configuration/ext.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/configuration/settings.rb	95.20% <100.00%> (+0.13%)	⬆️
lib/datadog/tracing/distributed/propagation.rb	93.47% <100.00%> (+1.37%)	⬆️
lib/datadog/tracing/trace_digest.rb	100.00% <100.00%> (ø)
...pec/datadog/tracing/configuration/settings_spec.rb	100.00% <100.00%> (ø)
...ec/datadog/tracing/distributed/propagation_spec.rb	100.00% <100.00%> (ø)
spec/datadog/tracing/trace_digest_spec.rb	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[ekump]

non-blocking: Is there any reason to be concerned about performance here?

[marcotc]

It's not really performance, but more about maintainability.
This code section would have to be duplicated here:

dd-trace-rb/lib/datadog/tracing/trace_digest.rb

Lines 122 to 140 in 8b6432a

	@span_id = span_id
	@span_name = span_name && span_name.dup.freeze
	@span_resource = span_resource && span_resource.dup.freeze
	@span_service = span_service && span_service.dup.freeze
	@span_type = span_type && span_type.dup.freeze
	@trace_distributed_tags = trace_distributed_tags && trace_distributed_tags.dup.freeze
	@trace_hostname = trace_hostname && trace_hostname.dup.freeze
	@trace_id = trace_id
	@trace_name = trace_name && trace_name.dup.freeze
	@trace_origin = trace_origin && trace_origin.dup.freeze
	@trace_process_id = trace_process_id
	@trace_resource = trace_resource && trace_resource.dup.freeze
	@trace_runtime_id = trace_runtime_id && trace_runtime_id.dup.freeze
	@trace_sampling_priority = trace_sampling_priority
	@trace_service = trace_service && trace_service.dup.freeze
	@trace_distributed_id = trace_distributed_id
	@trace_flags = trace_flags
	@trace_state = trace_state && trace_state.dup.freeze
	@trace_state_unknown_fields = trace_state_unknown_fields && trace_state_unknown_fields.dup.freeze

But it has to decide which fields can be used as is, or which ones need to be dup.freeze.

This is just to make sure we don't duplicate the logic and forget to copy some values but not others.