Fix exporter unittest occasional error

[Petrie]

A few days ago, I open a pr open-telemetry#2987 , try fix golangci-lint not working correctly. But the pr checks occur a error in unittest TestExporterShutdown/testClientStopHonorsTimeout for exporter grpc client Stop() function :exporters/otlp/otlptrace/otlptracegrpc/client.go#L130

I figure out in Stop(ctx context.Context) the select for <-ctx.Done() and acquired may reach at the same time. In select if acquired be exec first, The Stop(ctx context.Context) function may return nil . when Stop(ctx context.Context) function return nil, unittest TestExporterShutdown/testClientStopHonorsTimeout and TestExporterShutdown/testClientStopHonorsTimeout will failure.

This is a occasional error. you can recurrent it by this shell.

cd exporters/otlp/otlptrace/otlptracegrpc/
while true ; do go clean -testcache | go list \
| xargs go test -v -timeout 60s -race -run \
  TestExporterShutdown/testClientStopHonorsTimeout; sleep 0.5;  done;