long snapshotsRetrieved() {
        return snapshotsRetrieved.get();
    }
Retrieves metrics from a single Vespa node over http. To avoid unnecessary load on metrics
proxies, a cached snapshot per consumer is retained and served for a fixed TTL period.
Upon failure to retrieve metrics, an empty snapshot is cached.
<p>
This class assumes that the consumer id is a valid and existing one, which is already
ensured by the {@link ApplicationMetricsHandler}.

@author gjoranv
metrics-proxy/src/main/java/ai/vespa/metricsproxy/http/application/NodeMetricsClient.java