private void checkHealth(List<RemoteInstance> remoteInstances) {
        ClusterHealthStatus healthStatus = OAPNodeChecker.isHealth(remoteInstances);
        if (healthStatus.isHealth()) {
            this.healthChecker.health();
        } else {
            this.healthChecker.unHealth(healthStatus.getReason());
        }
    }
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