code: public void registerSince( @Nonnull final StatsType statsType, final long startInMillis ) {
          register( statsType, System.currentTimeMillis() - startInMillis );
      }
docs: A utility method that calculates the difference of the time
  between the given <code>startInMillis</code> and {@link System#currentTimeMillis()}
  and registers the difference via {@link #register(long)} for the probe of the given {@link StatsType}.
  @param statsType the specific execution type that is measured.
  @param startInMillis the time in millis that shall be subtracted from {@link System#currentTimeMillis()}.