Update the TracingKernelSubscriber example

[wadjei]

Since changes in open-telemetry/opentelemetry-php#760,
the original Kernel Listener example no longer works...

Following those changes, the stored Tracer instance gets deleted before the
application terminates so mainSpan can't be ended and none of the collected
trace info gets sent.
By refactoring to use TracerProvider instead, mainSpan's Tracer remains valid
during KernelEvents::TERMINATE so the trace session can be sent successfully.

[codecov]

Codecov Report

Merging #76 (3c8d9c1) into main (7b5c466) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main      #76   +/-   ##
=========================================
  Coverage     95.81%   95.81%           
  Complexity      182      182           
=========================================
  Files            18       18           
  Lines           454      454           
=========================================
  Hits            435      435           
  Misses           19       19           

Flag	Coverage Δ
7.4	95.81% <ø> (ø)
8.0	95.81% <ø> (ø)
8.1	95.81% <ø> (ø)

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[Nevay]

This should be fixed in the SDK bundle instead. TracerProvider::shutdown() should be called in Bundle::shutdown() and on exit (default Symfony HttpKernelRunner doesn't seem to call ::shutdown()).

Side note: There are some other problems with the example subscriber such as not detaching the created scope. I will see if I can create a PR over the weekend to fix the subscriber / add an actual implementation.