code: public final Mono<T> checkpoint(String description) {
  		return checkpoint(Objects.requireNonNull(description), false);
  	}
docs: Activate assembly marker for this particular {@link Mono} by giving it a description that
  will be reflected in the assembly traceback in case of an error upstream of the
  checkpoint. Note that unlike {@link #checkpoint()}, this doesn't create a
  filled stack trace, avoiding the main cost of the operator.
  However, as a trade-off the description must be unique enough for the user to find
  out where this Mono was assembled. If you only want a generic description, and
  still rely on the stack trace to find the assembly site, use the
  {@link #checkpoint(String, boolean)} variant.
  <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.
  
  @param description a unique enough description to include in the light assembly traceback.
  @return the assembly marked {@link Mono}