drop failed to exporter batches and return error when forcing flush a span processor

[paivagustavo]

This resolves a problem in the batch span processor and a nice-to-have error report on the ForceFlush() method.

Failed span batches are now dropped as expected. Quoting specs:

Failure - exporting failed. The batch must be dropped. For example, this can happen when the batch contains bad data and cannot be serialized.

BatchSpanProcessor.ForceFlush() method now correctly report export errors. Quoting specs:

ForceFlush SHOULD provide a way to let the caller know whether it succeeded, failed or timed out.

Fixes #1833

[codecov]

Codecov Report

Merging #1860 (0ad6cc6) into main (f6a9279) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #1860   +/-   ##
=====================================
  Coverage   78.6%   78.6%           
=====================================
  Files        137     137           
  Lines       7304    7305    +1     
=====================================
+ Hits        5743    5749    +6     
+ Misses      1316    1313    -3     
+ Partials     245     243    -2     

Impacted Files	Coverage Δ
sdk/trace/batch_span_processor.go	87.1% <100.0%> (+3.1%)	⬆️
exporters/trace/jaeger/jaeger.go	92.8% <0.0%> (+0.5%)	⬆️

[Aneurysm9]

It shouldn't matter with the batch being reset regardless of errors, but there's a line in processQueue() that should be updated. It's currently:

shouldExport := len(bsp.batch) == bsp.o.MaxExportBatchSize

and should probably be:

shouldExport := len(bsp.batch) >= bsp.o.MaxExportBatchSize