reactor-core/src/main/java/reactor/core/publisher/ParallelFlux.java
public final ParallelFlux<T> checkpoint() {
		AssemblySnapshot stacktrace = new AssemblySnapshot(null, Traces.callSiteSupplierFactory.get());
		return new ParallelFluxOnAssembly<>(this, stacktrace);
	}
public final ParallelFlux<T> checkpoint() {
		AssemblySnapshot stacktrace = new AssemblySnapshot(null, Traces.callSiteSupplierFactory.get());
		return new ParallelFluxOnAssembly<>(this, stacktrace);
	}
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}