private void updateRemoteInstances() {
        List<String> updatedInstances = new ArrayList<>(this.remoteInstanceMap.keySet());
        if (this.latestInstances.size() != updatedInstances.size() || !this.latestInstances.containsAll(updatedInstances)) {
            List<RemoteInstance> remoteInstances = new ArrayList<>(this.remoteInstanceMap.values());
            this.latestInstances = updatedInstances;
            checkHealth(remoteInstances);
            notifyWatchers(remoteInstances);
        }
    }
When a remote instance up/off line, will receive multi event according to the pod status.
To avoid notify the watchers too frequency, here use a `remoteInstanceMap` to cache them.
Only notify watchers once when the instances changed.
oap-server/server-cluster-plugin/cluster-kubernetes-plugin/src/main/java/org/apache/skywalking/oap/server/cluster/plugin/kubernetes/KubernetesCoordinator.java