Add intermediate transport layer between http/grpc client and exporter

[Nevay]

Allows removing duplication between OtlpGrpc and OtlpHttp, both exporters can now use the same implementation with differing underlying transport. Also simplifies implementing async exporters by simply swapping the transport implementation as all of the relevant logic is encapsulated in the TransportInterface.

Looking for feedback whether we want to go in this direction before adding tests - for now only tested with adapted make smoke-test-collector-integration.

$transportFactory = new PsrTransportFactory(
    Psr18ClientDiscovery::find(),
    Psr17FactoryDiscovery::findRequestFactory(),
    Psr17FactoryDiscovery::findStreamFactory(),
);
$exporter = new Exporter($transportFactory->create('http://collector:4318/v1/traces'));

$transportFactory = new GrpcTransportFactory();
$exporter = new Exporter($transportFactory->create('http://collector:4317/opentelemetry.proto.collector.trace.v1.TraceService/Export'));

[codecov]

Codecov Report

Merging #809 (bfa55b1) into main (ac3a35d) will decrease coverage by 0.46%.
The diff coverage is 68.33%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #809      +/-   ##
============================================
- Coverage     82.88%   82.41%   -0.47%     
- Complexity     1829     1885      +56     
============================================
  Files           225      233       +8     
  Lines          4697     4869     +172     
============================================
+ Hits           3893     4013     +120     
- Misses          804      856      +52     

Flag	Coverage Δ
7.4	82.42% <68.33%> (-0.47%)	⬇️
8.0	82.45% <68.33%> (-0.47%)	⬇️
8.1	82.45% <68.33%> (-0.47%)	⬇️

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

Impacted Files	Coverage Δ
src/Contrib/OtlpHttp/MetricExporter.php	0.00% <0.00%> (ø)
src/SDK/Common/Export/Stream/StreamTransport.php	0.00% <0.00%> (ø)
...DK/Common/Export/Stream/StreamTransportFactory.php	0.00% <0.00%> (ø)
src/SDK/Common/Future/NullCancellation.php	0.00% <0.00%> (ø)
src/Contrib/Grpc/GrpcTransportFactory.php	53.48% <53.48%> (ø)
src/Contrib/Grpc/GrpcTransport.php	56.66% <56.66%> (ø)
src/Contrib/Otlp/StreamMetricExporter.php	57.14% <58.33%> (+18.01%)	⬆️
src/Contrib/OtlpHttp/Exporter.php	83.67% <82.22%> (-10.33%)	⬇️
src/SDK/Common/Export/Http/PsrUtils.php	89.36% <89.36%> (ø)
src/SDK/Common/Export/Http/PsrTransport.php	98.27% <98.27%> (ø)
... and 10 more

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[brettmc]

I've had a look through this PR, I really like the approach. My vote is to proceed.

[Nevay]

Side note: This is only for BC / to avoid extending the scope of this PR. I don't think that the current usage of $endpointUrl as host is correct (should be the service host and not the new relic endpoint?), host and service.name should likely be fetched from the span resource instead.

[brettmc]

initial review, looks great. I made a couple of observations...

[brettmc]

This looks like it replaces #677 ?

[Nevay]

It replaces it partly / resolves the original issue.
The generic RetryPolicyInterface approach can still be implemented on top of this PR (the grpc transport would need a rewrite to use an explicit retry loop similar to the http transport).