Add a util function to ReactorUtils that makes it easier to create a span for Reactor / WebFlux

[adinauer]

Description

When using Reactor (e.g. when using Spring Boot WebFlux), the way to start a Sentry span that is also set as the current span is as follows:

NOTE: This code lacks error handling!

@RestController
public class TodoController {
  private final WebClient webClient;

  public TodoController(WebClient webClient) {
    this.webClient = webClient;
  }

  @GetMapping("/todo-webclient/{id}")
  Mono<Todo> todoWebClient(@PathVariable Long id) {
    return Mono.deferContextual(ctx -> getTodoMono(id).doFinally((signalType) -> {
        ISpan span = ctx.get("sentry-span");
        span.finish();
      }))
      .contextWrite(context -> {
        ISpan span = Sentry.getSpan().startChild("intermediate-span");
        span.makeCurrent();
        return context.put("sentry-span", span);
      });
  }

  private Mono<Todo> getTodoMono(Long id) {
    return webClient
      .get()
      .uri("https://jsonplaceholder.typicode.com/todos/{id}", id)
      .retrieve()
      .bodyToMono(Todo.class)
      .map(response -> response);
  }
}

We could make it easier by adding something like ReactorUtils.withSentrySpan that takes care of the Context manipulation and reading.

This issue shall serve to gauge interest in such a util function. Please 👍 if you would like to see this implemented.