Add TraceResponse propagator

[cedricziel]

Note: This introduces a new experimental package as traceresponse is currently a w3c editors' draft.

This package provides a Trace Context HTTP Response Headers Format
propagator to inject the current span context into Response datastructures.

The main goal is to allow client-side technology (Real User Monitoring, HTTP Clients) to record
the server side context in order to allow referencing or assuming it.

[codecov]

Codecov Report

Merging #143 (526fc48) into main (f897e94) will increase coverage by 0.85%.
The diff coverage is 100.00%.

❗ Current head 526fc48 differs from pull request most recent head 24632e7. Consider uploading reports for the commit 24632e7 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #143      +/-   ##
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+ Coverage     25.03%   25.88%   +0.85%     
- Complexity      315      319       +4     
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  Files            32       33       +1     
  Lines          1306     1321      +15     
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+ Hits            327      342      +15     
  Misses          979      979              

Flag	Coverage Δ
7.4	73.02% <100.00%> (+0.97%)	⬆️
8.0	30.28% <100.00%> (+0.94%)	⬆️
8.1	30.37% <100.00%> (+0.94%)	⬆️
8.2	25.90% <100.00%> (+0.85%)	⬆️

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Impacted Files	Coverage Δ
...tion/TraceResponse/src/TraceResponsePropagator.php	100.00% <100.00%> (ø)

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[brettmc]

A couple of small things from me, but this looks good.

We might need to think about how this propagator interacts with the tracecontext one; for example we current can globally configure a propagator which is then accessible from API\Globals::propagator(). The spec talks about a composite propagator - we don't have one yet (probably because it has not been needed before now)

[cedricziel]

We might need to think about how this propagator interacts with the tracecontext one; for example we current can globally configure a propagator which is then accessible from API\Globals::propagator(). The spec talks about a composite propagator - we don't have one yet (probably because it has not been needed before now)

True - the reason i didnt opt for this, is that traceresponse is strictly limited to responses and without a specific setter implementation, the propagator wont know.

My first approach was to explore in core to have Globals::responsePropagators(), but that seemed to be too invasive for an experimental package.

[cedricziel]

Note that I separated out Symfony (#146) and Slim (#147) auto propagation so we can discuss them separately after this is merged, gitsplit happened and the package has been created.