private Activity getActivity() {
        if (state == null) {
            throw new IllegalStateException("Trying to measure an interval having only one point.");
        }
        return state.activity;
    }
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