@Override
    public boolean equals(Object o) {
        if (o == null || getClass() != o.getClass()) return false;
        ContentNodeErrorStats that = (ContentNodeErrorStats) o;
        return Objects.equals(statsFromDistributors, that.statsFromDistributors);
    }
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