add instrumentation to PeriodicWorkerTask and InstantPeriodicWorkerTa…

[jack-berg]

Fix issue preventing activity from Flux.interval() from being linked to the transaction.

In an spring webflux set, this allows the following code to properly link the external activity:

@GetMapping("/reactiveFluxInterval")
Flux<String> handleReactiveWithFluxInterval() {
    return Flux.interval(Duration.ZERO, Duration.ofSeconds(1))
        .flatMap((Function<Long, Publisher<String>>) aLong -> WEB_CLIENT.get().retrieve().bodyToMono(String.class));
}