code: public final Mono<T> filter(final Predicate<? super T> tester) {
  		if (this instanceof Fuseable) {
  			return onAssembly(new MonoFilterFuseable<>(this, tester));
  		}
  		return onAssembly(new MonoFilter<>(this, tester));
  	}
docs: If this {@link Mono} is valued, test the result and replay it if predicate returns true.
  Otherwise complete without value.
  
  <p>
  <img class="marble" src="doc-files/marbles/filterForMono.svg" alt="">
  
  @reactor.discard This operator discards the element if it does not match the filter. It
  also discards upon cancellation or error triggered by a data signal.
  
  @param tester the predicate to evaluate
  
  @return a filtered {@link Mono}