code: public void streamClosed(Status status) {
      if (closed.compareAndSet(false, true)) {
        for (StreamTracer tracer : tracers) {
          tracer.streamClosed(status);
        }
      }
    }
docs: See {@link StreamTracer#streamClosed}. This may be called multiple times, and only the first
  value will be taken.
  
  <p>Called from abstract stream implementations.