void clear() {
      cacheNames.forEach(cacheMetricsCollector::removeCache);
      cacheNames.clear();
      collectorRegistered.set(false);
    }
Since Prometheus provides only one collector for all Guava caches, we only need to register
that collector once when the first Besu Guava cache collector is created, and unregister it
when the last is unregistered, so we have this context to keep track of that and also manage
the observations stream.
metrics/core/src/main/java/org/hyperledger/besu/metrics/prometheus/PrometheusGuavaCache.java