@Override public String toString() {
    return "Throttled(" + delegate + ")";
  }
To simplify code, this doesn't actually invoke the underlying {@link #execute()} method. This
is ok because in almost all cases, doing so would imply invoking {@link #enqueue(Callback)}
anyway.
zipkin-server/src/main/java/zipkin2/server/internal/throttle/ThrottledCall.java