code: public final ParallelFlux<T> checkpoint() {
  		AssemblySnapshot stacktrace = new AssemblySnapshot(null, Traces.callSiteSupplierFactory.get());
  		return new ParallelFluxOnAssembly<>(this, stacktrace);
  	}
docs: Activate assembly tracing for this particular {@link ParallelFlux}, in case of an
  error upstream of the checkpoint. Tracing incurs the cost of an exception stack trace
  creation.
  <p>
  It should be placed towards the end of the reactive chain, as errors
  triggered downstream of it cannot be observed and augmented with assembly trace.
  
  @return the assembly tracing {@link ParallelFlux}