private static long valueOrZero(Long valOrNull) {
        return valOrNull != null ? valOrNull : 0;
    }
Encapsulates response-related information received by _one_ particular distributor
related to its communication with _one_ particular content node. This is a superset
of information than what strictly covers error statistics (by including the total
number of responses, which includes successes), hence the name ResponseStats rather
than just "ErrorStats" or similar. Error-specific statistics may be further extracted
from this superset.

@author vekterli
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/hostinfo/ResponseStats.java