Fix reactor context propagation in latest reactor

[lmolkova]

Fixes #4372 introduced in #4159

The newer reactor does not support a hack dummy.flatMap(i -> publisher) to force publisher in onNext (i.e. with current() span)

It seems there is optimization or some different path that causes the below branch to run, skipping context instrumentation

opentelemetry-java-instrumentation/instrumentation/reactor-3.1/library/src/main/java/io/opentelemetry/instrumentation/reactor/TracingOperator.java

Lines 91 to 94 in 51cf64b

	// if Flux/Mono #just, #empty, #error
	if (publisher instanceof Fuseable.ScalarCallable) {
	return sub;
	}

One way to solve it would be to enable context propagation for scalars, but it probably would be bad for performance.

Instead, I went down the road of creating a small custom Mono impl instead of dummy that implements context propagation in subscribe call.

[trask]

@lmolkova looks like ./gradlew :instrumentation:reactor-3.1:library:codenarcTest is failing, usually for me that means I used semi-colons in groovy code 😢