public Optional<String> getDimensionValue(String dimension) {
            return Optional.ofNullable(dimensions.get(dimension));
        }
Get the metric value whose dimensions MUST MATCH the given dimensions map.
To require the metric to NOT have a dimension key, set it's value to null.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/hostinfo/Metrics.java