title: Sleuth tracer.currentSpan() coming as empty in WebFlux WebFilter
body: <p>How Can I add Trace Id in the response headers in reactive WebFlux context ? I have tried with the below code but the span is always coming as null. Sleuth dependency version is 3.0.3 and Spring Boot version is 2.5.4</p>
  <pre><code>import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.cloud.sleuth.Span;
  import org.springframework.cloud.sleuth.Tracer;
  import org.springframework.core.Ordered;
  import org.springframework.core.annotation.Order;
  import org.springframework.http.server.reactive.ServerHttpResponse;
  import org.springframework.stereotype.Component;
  import org.springframework.web.server.ServerWebExchange;
  import org.springframework.web.server.WebFilter;
  import org.springframework.web.server.WebFilterChain;
  import reactor.core.publisher.Mono;
  
  @Component
  public class GatewaySleuthFilter implements WebFilter {
  
      @Autowired
      Tracer tracer;
      
      @Override
        public Mono&lt;Void&gt; filter(ServerWebExchange exchange,
                                 WebFilterChain chain) {
          ServerHttpResponse response = exchange.getResponse();
          response.beforeCommit(() -&gt; {
            Span span = tracer.currentSpan();
            if (span != null) {
              exchange.getResponse().getHeaders().add(&quot;trace-id&quot;, span.context().traceId());
              exchange.getResponse().getHeaders().add(&quot;span-id&quot;, span.context().spanId());
            }
            return Mono.empty();
          });
          return chain.filter(exchange);
        }
  
  }
  </code></pre>