Fix#5384
Run of `go test -count=1000000 -run="TestBatchProcessor/DroppedLogs"`
**Before**:
Failed with either `Condition never satisfied` or panic
**After**:
Passed
First, bytes.Buffer is not thread-safe, so writing log and reading
(`bytes.String()`) caused panic. Added `concurrentBuffer`
Second, fixed flaky test with 4 records:
1. Record goes to `testExporter.Export` function and blocks in this
function because of `ExportTrigger`
(19ee6d4775/sdk/log/exporter_test.go (L87))
2. Record goes to `bufferExporter.input`
(7c5e64cccc/sdk/log/exporter.go (L129))
3. Record goes to `BatchProcessor.q` queue and it could not be enqueued
to export, because `bufferExporter.input` is full
4. Record goes to `BatchProcessor.q` and because of overfill, drops
third record
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Co-authored-by: Sam Xie <sam@samxie.me>
982e96dd9b