Span processor exporter updates

[Blacksmoke16]

• Update docs on various types/methods to point to related spec sections
• Rename Exporter to SpanExporterInterface
  • Was missed in Reorganize types #434
• Update various shutdown and forceFlush methods to return bool
  • Spec says they should provide a way to differentiate between pass/fail
  • The spec also says to support a timeout, but I'm not sure how you'd do that in PHP given it's single threaded and doesn't have async stuff like other langs
• Implemented NoopTracer and NoopSpanBuilder
  • Is returned when you call #getTracer on a shutdown TracerProvider
• Ensure #shutdown also calls #forceFlush as per the spec
• Rename SpanMultiProcessor to MultiSpanProcessor to better match naming of other processors

TODO:

• Want to update some more tests, so starting this off as a draft

Resolves #270

[codecov]

Codecov Report

Merging #439 (9c6147c) into main (c8756f1) will decrease coverage by 1.69%.
The diff coverage is 54.23%.

@@             Coverage Diff              @@
##               main     #439      +/-   ##
============================================
- Coverage     92.84%   91.14%   -1.70%     
- Complexity      781      804      +23     
============================================
  Files            74       76       +2     
  Lines          1929     1977      +48     
============================================
+ Hits           1791     1802      +11     
- Misses          138      175      +37     

Impacted Files	Coverage Δ
src/SDK/Trace/NoopSpanBuilder.php	0.00% <0.00%> (ø)
src/SDK/Trace/NoopTracer.php	0.00% <0.00%> (ø)
src/SDK/Trace/SpanProcessor/NoopSpanProcessor.php	60.00% <0.00%> (-6.67%)	⬇️
src/SDK/Trace/TracerProvider.php	67.85% <12.50%> (-7.15%)	⬇️
src/SDK/Trace/SpanProcessor/MultiSpanProcessor.php	20.00% <16.66%> (ø)
src/SDK/Trace/TracerSharedState.php	92.85% <50.00%> (+3.20%)	⬆️
src/Contrib/ZipkinToNewrelic/Exporter.php	90.38% <75.00%> (+0.18%)	⬆️
...rc/SDK/Trace/SpanProcessor/SimpleSpanProcessor.php	94.11% <88.88%> (-5.89%)	⬇️
src/Contrib/OtlpHttp/Exporter.php	97.14% <90.00%> (+0.04%)	⬆️
src/SDK/Trace/SpanProcessor/BatchSpanProcessor.php	95.55% <90.90%> (-1.95%)	⬇️
... and 3 more

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