Add alternative instrumentation abstraction

[Nevay]

Adds global (context-scoped) tracer provider & co, intended to replace the current InstrumentationInterface.

$scope = InstrumentationConfigurator::create()
    ->withTracerProvider($tracerProvider)
    ->withPropagator($propagator)
    ->activate();

try {
    // ...
} finally {
    $scope->detach();
}

$instrumentation = new Instrumentation('io.opentelemetry.contrib.php');
// ...
$span = $instrumentation
    ->tracer() // returns a tracer for the currently active tracer provider
    ->spanBuilder('test')
    ->startSpan()

[codecov]

Codecov Report

Merging #822 (d7c308d) into main (f73a835) will increase coverage by 0.22%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #822      +/-   ##
============================================
+ Coverage     81.52%   81.75%   +0.22%     
- Complexity     1925     1947      +22     
============================================
  Files           239      243       +4     
  Lines          5008     5069      +61     
============================================
+ Hits           4083     4144      +61     
  Misses          925      925              

Flag	Coverage Δ
7.4	81.23% <90.16%> (+0.10%)	⬆️
8.0	81.75% <95.08%> (+0.16%)	⬆️
8.1	81.89% <95.08%> (+0.16%)	⬆️

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

Impacted Files	Coverage Δ
...I/Common/Instrumentation/CachedInstrumentation.php	100.00% <100.00%> (ø)
src/API/Common/Instrumentation/Configurator.php	100.00% <100.00%> (ø)
src/API/Common/Instrumentation/ContextKeys.php	100.00% <100.00%> (ø)
src/API/Common/Instrumentation/Globals.php	100.00% <100.00%> (ø)

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[Nevay]

If we merge this: SpanProcessors and MetricReaders should run exports with the Context they were created in.

[Nevay]

Should this implement ImplicitContextKeyedInterface?

[brettmc]

@Nevay happy for this to be merged?

[Nevay]

Should be ready after a rebase.