reactor-core/src/main/java/reactor/core/publisher/Mono.java
public final Mono<T> checkpoint(String description) {
		return checkpoint(Objects.requireNonNull(description), false);
	}
public final Mono<T> checkpoint(String description) {
		return checkpoint(Objects.requireNonNull(description), false);
	}
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}