@Override
    long getExpiresAtMs() {
      latch.countDown();
      return super.getExpiresAtMs();
    }
Mock {@link IndexValue} with a latch to act as a probe to inform us about the state of the newEntryQueue in
{@link BlobStoreStats}.
ambry-store/src/test/java/com/github/ambry/store/BlobStoreStatsTest.java