sendSegment is not thread-safe

[liberize]

sendSegment is not thread-safe because GrpcAsyncSegmentReporterStream::clearPendingMessages is not
which causes SIGABRT with proto_buffer_writer.h:65 assertion failed: !byte_buffer->Valid()

[wu-sheng]

@Shikugawa Do we open multiple threads for sending the segment?

[Shikugawa]

It won't be a problem on Envoy integration so I ignored this problem although I already know about it. But when we use it as SDK, It will be a critical problem. This problem is not directly related with Envoy integration. So I will submit a patch for 0.2.0.

[Shikugawa]

@liberize After all we don't have to allow sharing tracer between threads because it should be lock-free. Have you considered this to have per thread?

[wu-sheng]

@Shikugawa Tracer as per thread is fine, if it is designed in this way, but notice, the sender and grpc channel should be shared across threads, typically using a memory queue to buffer.

[xlj]

I have the same problem. this problem occur in my test demo. demo has only one thread.
this is my demo code:
`#include
#include

#include "cpp2sky/propagation.h"
#include "cpp2sky/tracer.h"
#include "cpp2sky/tracing_context.h"
#include "cpp2sky/well_known_names.h"
using namespace cpp2sky;
TracerConfig config;

void DoTest(int num){
std::fprintf(stdout, "start\n");
config.set_instance_name("node_0");
config.set_service_name("mesh");
config.set_address("0.0.0.0:11800");
TracerPtr _Kcbp_tracer = createInsecureGrpcTracer(config);
std::fprintf(stdout, "start1\n");
while (num-- > 0) {
std::fprintf(stdout, "num:%d\n", num);
auto tracing_context = _Kcbp_tracer->newContext();
{
std::shared_ptr spExitAf = std::make_shared(tracing_context, spSpan->get(), "KCBP_SKYNAME_BpcallAfter");

        auto span = tracing_context->createEntrySpan();
        span->startSpan("/grpc_client.Greeter/handle");
        if (tracing_context != nullptr) {
            span->endSpan();
        }
    }
    bool bRet = _Kcbp_tracer->report(std::move(tracing_context));
}

std::fprintf(stdout, "end\n");

}

int main() {
std::fprintf(stdout, "main\n");
DoTest(3000);
return 0;
}`

the cpp2sky branch is main.
gcc 4.9.0