avoid calling v4 api

[zmstone]

this version probing will generate some suspicious logs in emqx v5.

emqx-exporter/client/cluster.go

Lines 53 to 84 in 16b7648

	func (c *cluster) checkNodes() {
	httpClient := getHTTPClient(*emqxNodes)
	var currentVersion string
	for {
	var client client
	var err4, err5 error
	client = &cluster4x{client: httpClient}
	_, err4 = client.getClusterStatus()
	if err4 != nil {
	client = &cluster5x{client: httpClient}
	_, err5 = client.getClusterStatus()
	}
	if err4 != nil && err5 != nil {
	_ = level.Warn(c.logger).Log("check nodes", "couldn't get node info", "addr", *emqxNodes,
	"err4", err4.Error(), "err5", err5.Error())
	client = nil
	} else if currentVersion != client.getVersion() {
	currentVersion = client.getVersion()
	_ = level.Info(c.logger).Log("ClusterVersion", currentVersion)
	}
	c.nodeLock.Lock()
	c.client = client
	c.nodeLock.Unlock()
	select {
	case <-context.Background().Done():
	return
	case <-time.After(5 * time.Second):
	}
	}
	}

an alternative is to try to call the /status endpoint with ?format=json, in v4 it will return text (ignore the json arg).
in v5 it returns a json like below

curl -s 'http://127.0.0.1:18083/status?format=json'
{"rel_vsn":"e5.1.1","node_name":"emqx@127.0.0.1","broker_status":"started","app_status":"running"}