sdk/trace: Refine context cancellation in batchSpanProcessor.ForceFlush

[pellared]

Fixes #4368

The issue is that the flush request can be send and processed before <-ctx.Done() is handled. Now we just check if context is not canceled before doing anything else to have more deterministic behavior.

After 0c8d9fa:

$ go test -race -count=100000 -run TestBatchSpanProcessorForceFlushTimeout
--- FAIL: TestBatchSpanProcessorForceFlushTimeout (0.00s)
    batch_span_processor_test.go:571: expected "context deadline exceeded" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushTimeout (0.00s)
    batch_span_processor_test.go:571: expected "context deadline exceeded" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushTimeout (0.01s)
    batch_span_processor_test.go:571: expected "context deadline exceeded" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushTimeout (0.00s)
    batch_span_processor_test.go:571: expected "context deadline exceeded" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushTimeout (0.00s)
    batch_span_processor_test.go:571: expected "context deadline exceeded" error, got <nil>
FAIL
exit status 1
FAIL    go.opentelemetry.io/otel/sdk/trace      72.959s

$ go test -race -count=100000 -run TestBatchSpanProcessorForceFlushCancellation
--- FAIL: TestBatchSpanProcessorForceFlushCancellation (0.00s)
    batch_span_processor_test.go:559: expected "context canceled" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushCancellation (0.00s)
    batch_span_processor_test.go:559: expected "context canceled" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushCancellation (0.00s)
    batch_span_processor_test.go:559: expected "context canceled" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushCancellation (0.00s)
    batch_span_processor_test.go:559: expected "context canceled" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushCancellation (0.00s)
    batch_span_processor_test.go:559: expected "context canceled" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushCancellation (0.00s)
    batch_span_processor_test.go:559: expected "context canceled" error, got <nil>
--- FAIL: TestBatchSpanProcessorForceFlushCancellation (0.00s)
    batch_span_processor_test.go:559: expected "context canceled" error, got <nil>
FAIL
exit status 1
FAIL    go.opentelemetry.io/otel/sdk/trace      55.897s

After 1533304:

$ go test -race -count=100000 -run TestBatchSpanProcessorForceFlushTimeout
PASS
ok      go.opentelemetry.io/otel/sdk/trace      71.522s

$ go test -race -count=100000 -run TestBatchSpanProcessorForceFlushCancellation
PASS
ok      go.opentelemetry.io/otel/sdk/trace      54.733s

[codecov]

Codecov Report

Merging #4369 (bc6be57) into main (b4264c5) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4369     +/-   ##
=======================================
- Coverage   83.4%   83.4%   -0.1%     
=======================================
  Files        184     184             
  Lines      14380   14383      +3     
=======================================
- Hits       11999   11998      -1     
- Misses      2153    2157      +4     
  Partials     228     228             

Files Changed	Coverage Δ
sdk/trace/batch_span_processor.go	78.4% <100.0%> (-1.5%)	⬇️