feat: Add experimental response propagator interface

[jerrytfleung]

Issue

Currently there are 2 response propagators (ServerTimingPropagator & TraceResponsePropagator) in otel-php to propagate information to response. However, they are called explicitly in different instrumentations (e.g. Slim, Symfony, etc). As a result, vendors are unable to propagate their specific information to response.

Proposal

With the ongoing otel spec PR (1355, 3825) and they are still under review, I am proposing an experimental response propagator interface so as to allow vendor to propagate their specific response.
Once this PR is in, both ServerTimingPropagator and TraceResponsePropagator can implement the same ResponsePropagatorInterface and the following code in the instrumentation library could be replaced

// Propagate server-timing header to response, if ServerTimingPropagator is present
if (class_exists('OpenTelemetry\Contrib\Propagation\ServerTiming\ServerTimingPropagator')) {
    $prop = new \OpenTelemetry\Contrib\Propagation\ServerTiming\ServerTimingPropagator();
    $prop->inject($response, ResponsePropagationSetter::instance(), $scope->context());
}
// Propagate traceresponse header to response, if TraceResponsePropagator is present
if (class_exists('OpenTelemetry\Contrib\Propagation\TraceResponse\TraceResponsePropagator')) {
    $prop = new \OpenTelemetry\Contrib\Propagation\TraceResponse\TraceResponsePropagator();
    $prop->inject($response, ResponsePropagationSetter::instance(), $scope->context());
}

by

$prop = Globals::responsePropagator();
$prop->inject($response, ResponsePropagationSetter::instance(), $scope->context());

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[codecov]

Codecov Report

❌ Patch coverage is 48.78049% with 63 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.10%. Comparing base (2f3e214) to head (e9e72fb).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
.../Config/SDK/ComponentProvider/OpenTelemetrySdk.php	0.00%	31 Missing ⚠️
...rovider/Propagator/ResponsePropagatorComposite.php	0.00%	7 Missing ⚠️
.../SDK/Propagation/LateBindingResponsePropagator.php	0.00%	6 Missing ⚠️
src/SDK/Propagation/ResponsePropagatorFactory.php	64.70%	6 Missing ⚠️
src/SDK/Registry.php	37.50%	5 Missing ⚠️
src/SDK/Propagation/_register.php	0.00%	4 Missing ⚠️
src/SDK/SdkAutoloader.php	75.00%	3 Missing ⚠️
src/Context/Propagation/NoopResponsePropagator.php	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1712      +/-   ##
============================================
- Coverage     68.37%   68.10%   -0.28%     
- Complexity     2878     2919      +41     
============================================
  Files           430      435       +5     
  Lines          8757     8876     +119     
============================================
+ Hits           5988     6045      +57     
- Misses         2769     2831      +62     

Flag	Coverage Δ
8.1	67.83% <48.78%> (-0.31%)	⬇️
8.2	68.03% <48.78%> (-0.30%)	⬇️
8.3	68.07% <48.78%> (-0.27%)	⬇️
8.4	68.01% <48.78%> (-0.32%)	⬇️
8.5	68.07% <48.78%> (-0.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/API/Globals.php	100.00% <100.00%> (ø)
...PI/Instrumentation/AutoInstrumentation/Context.php	100.00% <ø> (ø)
src/API/Instrumentation/Configurator.php	100.00% <100.00%> (ø)
src/API/Instrumentation/ContextKeys.php	100.00% <100.00%> (ø)
src/API/Instrumentation/InstrumentationTrait.php	95.91% <100.00%> (+0.46%)	⬆️
...rc/Context/Propagation/MultiResponsePropagator.php	100.00% <100.00%> (ø)
src/SDK/Sdk.php	100.00% <100.00%> (ø)
src/SDK/SdkBuilder.php	100.00% <100.00%> (ø)
src/Context/Propagation/NoopResponsePropagator.php	83.33% <83.33%> (ø)
src/SDK/SdkAutoloader.php	91.97% <75.00%> (-1.73%)	⬇️
... and 6 more

... and 3 files with indirect coverage changes

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[brettmc]

This looks pretty sensible. Can you add an example of how it's used?
I think it would be more ergonomic if the response propagator was obtained from Globals like other propagators are. That's an API change, but it's a php-specific part of the API that's not strictly defined in spec.
I think it would be worth linking to the relevant issues/PRs from code, for future reference.

[brettmc]

Hmm. I agree that ideally this is how it would work, and the config spec allows extra entries here, but I think we should be cautious about adding non-spec entries here. At the least, it should be experimental_response_propagator. My main concern is incompatibility if/when something is added to spec.
The config repo may accept a PR adding it as experimental and optional?

[jerrytfleung]

Done. Changed to use response_propagator/development

[brettmc]

This looks pretty good. Most of my review comments are about not over-committing or doing things that might conflict with future spec changes. We need to be cautious while moving ahead of the spec.

[jerrytfleung]

This looks pretty good. Most of my review comments are about not over-committing or doing things that might conflict with future spec changes. We need to be cautious while moving ahead of the spec.

Yes, it is quite a lot of files change if adding the interface to Globals. While changing the config from response_propagator to response_propagator/development and using OTEL_EXPERIMENTAL_RESPONSE_PROPAGATORS, I think we could adapt to the future spec changes.

[brettmc]

I think this is good to go - just one minor tidy up thing please