Add self-diagnostic metrics to BatchSpanProcessor

[Nevay]

No description provided.

[codecov]

Codecov Report

Merging #828 (7dd3d5a) into main (7940773) will decrease coverage by 0.98%.
The diff coverage is 43.06%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #828      +/-   ##
============================================
- Coverage     82.51%   81.52%   -0.99%     
- Complexity     1888     1925      +37     
============================================
  Files           234      239       +5     
  Lines          4908     5008     +100     
============================================
+ Hits           4050     4083      +33     
- Misses          858      925      +67     

Flag	Coverage Δ
7.4	81.12% <32.16%> (-1.41%)	⬇️
8.0	81.58% <43.21%> (-0.97%)	⬇️
8.1	81.73% <45.26%> (-0.82%)	⬇️

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Impacted Files	Coverage Δ
src/Contrib/Otlp/MetricExporter.php	0.00% <0.00%> (ø)
src/Contrib/Otlp/ProtobufSerializer.php	0.00% <0.00%> (ø)
src/Contrib/Otlp/SpanExporter.php	0.00% <0.00%> (ø)
src/Contrib/OtlpHttp/MetricExporter.php	0.00% <0.00%> (ø)
src/SDK/Common/Export/Http/PsrTransportFactory.php	100.00% <ø> (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/Contrib/Grpc/GrpcTransport.php	50.00% <33.33%> (-6.67%)	⬇️
...SDK/Common/Attribute/FilteredAttributesBuilder.php	56.52% <56.52%> (ø)
src/SDK/Common/Export/Http/PsrTransport.php	95.16% <60.00%> (-3.15%)	⬇️
... and 13 more

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[bobstrecansky]

@Nevay - this looks ready, just has a merge conflict that needs to be remediated.