public void addErrorStatsFrom(int distributorIndex, ContentNodeErrorStats stats) {
        mergeErrorStatsFrom(distributorIndex, stats, 1);
    }
A sparse mapping from distributors to error stats observed <em>by</em> that distributor
<em>towards</em> one particular content node (i.e. all distributor observations are for
the same content node). If no errors have been observed by a distributor, it will not
have an entry in this mapping, hence the sparseness.

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