title: add custom baggage to current span and accessed by log MDC
body: <p>i'm trying to add additional Baggage to the existing span on a HTTP server, i want to add a path variable to the span to be accessed from log MDC and to be propagated on the wire to the next server i call via http or kafka.</p>
  <p>my setup : spring cloud sleuth Hoxton.SR5 and spring boot 2.2.5</p>
  <p>i tried adding the following setup and configuration:</p>
  <pre><code>spring:
    sleuth:
      propagation-keys: context-id, context-type
      log:
        slf4j:
          whitelisted-mdc-keys: context-id, context-type
  </code></pre>
  <p>and added http interceptor :</p>
  <pre><code>public class HttpContextInterceptor implements HandlerInterceptor {
  
      
      private final Tracer tracer;
      private final HttpContextSupplier httpContextSupplier;
  
      @Override
      public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
          if (httpContextSupplier != null) {
              addContext(request, handler);
          }
          return true;
      }
  
      private void addContext(HttpServletRequest request, Object handler) {
          final Context context = httpContextSupplier.getContext(request);
          if (!StringUtils.isEmpty(context.getContextId())) {
              ExtraFieldPropagation.set(tracer.currentSpan().context(), TracingHeadersConsts.HEADER_CONTEXT_ID, context.getContextId());
          }
          if (!StringUtils.isEmpty(context.getContextType())) {
              ExtraFieldPropagation.set(tracer.currentSpan().context(), TracingHeadersConsts.HEADER_CONTEXT_TYPE, context.getContextType());
          }
      }
  }
  </code></pre>
  <p>and http filter to affect the current span(according to the spring docs)</p>
  <pre><code>public class TracingFilter extends OncePerRequestFilter {
  
      private final Tracer tracer;
  
      @Override
      protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
          try (Tracer.SpanInScope ws = tracer.withSpanInScope(tracer.currentSpan())){
              filterChain.doFilter(request, response);
          }
      }
  }
  </code></pre>
  <p><strong>the problem</strong> is the logs doesn't contain my custom context-id, context-type, although is see it in the span context.</p>
  <p>what i'm missing ?</p>