Add option in BatchSpanProcessor to support multiple pending exports

[trask]

Resolves #4264

This is a third option that I like better than both #4245 and #4246.

[codecov[bot]]

Codecov Report

Merging #4280 (1ccf507) into main (14ffacd) will increase coverage by 0.00%. The diff coverage is 95.65%.

@@            Coverage Diff            @@
##               main    #4280   +/-   ##
=========================================
  Coverage     89.88%   89.88%           
- Complexity     4844     4847    +3     
=========================================
  Files           568      568           
  Lines         15063    15081   +18     
  Branches       1427     1429    +2     
=========================================
+ Hits          13539    13556   +17     
  Misses         1060     1060           
- Partials        464      465    +1     

Impacted Files	Coverage Δ
...ry/sdk/trace/export/BatchSpanProcessorBuilder.java	94.73% <80.00%> (-2.24%)	:arrow_down:
...telemetry/sdk/trace/export/BatchSpanProcessor.java	91.02% <100.00%> (+0.11%)	:arrow_up:
...pentelemetry/sdk/common/CompletableResultCode.java	100.00% <0.00%> (+1.58%)	:arrow_up:

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[trask]

discussed with @anuraaga, I'm not in a rush for this change, will open a spec issue to discuss adding this setting to the spec

[github-actions[bot]]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[trask]

@open-telemetry/java-maintainers can you apply the prototype label to this (if you agree)?