@Bean ThreadLocalSpan threadLocalSpan(Tracing tracing) {
    return ThreadLocalSpan.create(tracing.tracer());
  }
There's no attribute namespace shared across request and response. Hence, we need to save off a
reference to the span in scope, so that we can close it in the response.
zipkin-server/src/main/java/zipkin2/server/internal/brave/ZipkinSelfTracingConfiguration.java