private static OptionalDouble optionalDouble(Inspector field) {
        return field.valid() ? OptionalDouble.of(field.asDouble()) : OptionalDouble.empty();
    }
Call the metrics API on each host and aggregate the metrics
into a single value, grouped by cluster.
configserver/src/main/java/com/yahoo/vespa/config/server/metrics/ClusterDeploymentMetricsRetriever.java