reactor-core/src/main/java/reactor/core/publisher/Mono.java
public final Mono<T> tag(String key, String value) {
		return MonoName.createOrAppend(this, key, value);
	}
public final Mono<T> tag(String key, String value) {
		return MonoName.createOrAppend(this, key, value);
	}
Tag this mono with a key/value pair. These can be retrieved as a {@link Set} of
all tags throughout the publisher chain by using {@link Scannable#tags()} (as
traversed
by {@link Scannable#parents()}).

@param key a tag key
@param value a tag value
@return the same sequence, but bearing tags