void addInvoking(Activity activity, long time) {
            Long storedTillNow = invoking.get(activity);
            long tillNow = getTime(storedTillNow);
            invoking.put(activity, Long.valueOf(tillNow + time));
        }
A container for storing time stamps throughout the lifetime of an Execution instance.

<p>Check state both when entering and exiting, to allow for arbitrary
new queries anywhere inside a search chain.

@author Steinar Knutsen
container-search/src/main/java/com/yahoo/search/statistics/TimeTracker.java