private LocalInstance<AGGREGATOR, SAMPLE> getOrCreateLocal() {
        LocalInstance<AGGREGATOR, SAMPLE> current = local.get();
        if (current == null) {
            current = new LocalInstance<>(updater);
            local.set(current);
        }
        return current;
    }
Get a view of the current data. This requires this ThreadLocalDirectory
to have been instantiated with an updater implementing ObservableUpdater.

@return a list of a copy of the current data in all producer threads
@throws IllegalStateException if the updater does not implement {@link ObservableUpdater}
vespajlib/src/main/java/com/yahoo/concurrent/ThreadLocalDirectory.java