code: public final Mono<T> metrics() {
  		if (!Metrics.isInstrumentationAvailable()) {
  			return this;
  		}
  
  		if (this instanceof Fuseable) {
  			return onAssembly(new MonoMetricsFuseable<>(this));
  		}
  		return onAssembly(new MonoMetrics<>(this));
  	}
docs: Activate metrics for this sequence, provided there is an instrumentation facade
  on the classpath (otherwise this method is a pure no-op).
  <p>
  Metrics are gathered on {@link Subscriber} events, and it is recommended to also
  {@link #name(String) name} (and optionally {@link #tag(String, String) tag}) the
  sequence.
  
  @return an instrumented {@link Mono}