public long firstFill() {
        for (Tag t : tags) {
            if (t.activity == Activity.FILL) {
                return t.start;
            }
        }
        return 0L;
    }
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