Add ServerTiming response propagator

[cedricziel]

This adds a response propagator for server-timings headers.

Server-Timing headers are especially useful as they are accessible for client side technology like RUM libraries. This is true even for the initial page-load. This makes server-timing the perfect vehicle to send the trace-id to the client for context-assumption, even without nasty hacks like meta html elements.

This approach has been demonstrated by vendors like Splunk and Instana. As there is currently no response propagation format agreed on in OpenTelemetry, the new package adds one as well. In a later iteration, these should optimally be backed by spec and then unified. For now, the interface is duplicated to avoid yet another package to be created.

Note to reviewers: The Symfony tests are failing, because the package referenced doesn't exist yet.

[codecov]

Codecov Report

Merging #213 (5b3801c) into main (f3e21f4) will increase coverage by 1.80%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #213      +/-   ##
============================================
+ Coverage     32.02%   33.82%   +1.80%     
+ Complexity      878      853      -25     
============================================
  Files            79       76       -3     
  Lines          3360     3225     -135     
============================================
+ Hits           1076     1091      +15     
+ Misses         2284     2134     -150     

Flag	Coverage Δ
7.4	46.73% <100.00%> (+0.38%)	⬆️
8.0	33.81% <100.00%> (+1.94%)	⬆️
8.1	33.84% <100.00%> (+1.94%)	⬆️
8.2	33.77% <100.00%> (+1.80%)	⬆️

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Files	Coverage Δ
...gation/ServerTiming/src/ServerTimingPropagator.php	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

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[cedricziel]

@brettmc the TextMapPropagator and the way it is used currently for requests is not suited for ServerTiming propagation.

If we registered ServerTimingPropagator there and used Globals::propagator()->inject(), not just server-timings would be propagated into the response, but also the other configured propagation formats, which is not the desired behavior. Same way, if server-timings was a regularly activated propagator, it would be propagated forward, which is also not the desired behavior.

What I thought about, is to amend the core to offer a dedicated method for response propagators:

// register response propagator in _register.php
\OpenTelemetry\SDK\Registry::registerTextMapResponsePropagator(
   'server-timing',
    ServerTimingPropagator::getInstance()
);

// get a composite propagator with _just_ response propagators
Globals::responsePropagator()->inject($carrier);

[brettmc]

What I thought about, is to amend the core to offer a dedicated method for response propagators

Yes, you're absolutely right. Are we confident enough in the idea of a response propagator to attempt that in core? I feel like it's something that should be spec-driven.

[brettmc]

LGTM - I think a future TODO to formalize in core the idea of a response propagator, possibly based on a spec enhancement.

[cedricziel]

I feel like it's something that should be spec-driven.

💯

@jpkrohling and I want to propose a spec amendment somewhere in the next 3 months

[brettmc]

@cedricziel one thing missing is an entry in .gitsplit.yaml for the new package.
I've created https://github.com/opentelemetry-php/contrib-propagator-server-timing as the target repo

[cedricziel]

@brettmc pushed!