Only initialize service URL once (#16044) (#16073)

* Only add vertices param to URL if it doesn't already exist

* Adding CHANGELOG entry

* Use sync.Once instead

CHANGELOG.next.asciidoc

@@ -116,6 +116,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Use RFC3339 format for timestamps collected using the SQL module. {pull}15847[15847]
 - Avoid parsing errors returned from prometheus endpoints. {pull}15712[15712]
 - Add dedot for cloudwatch metric name. {issue}15916[15916] {pull}15917[15917]
+- Fixed issue `logstash-xpack` module suddenly ceasing to monitor Logstash. {issue}15974[15974] {pull}16044[16044]
 
 *Packetbeat*
 

metricbeat/module/logstash/node_stats/node_stats.go

@@ -18,6 +18,8 @@
 package node_stats
 
 import (
+	"sync"
+
 	"github.com/elastic/beats/metricbeat/mb"
 	"github.com/elastic/beats/metricbeat/mb/parse"
 	"github.com/elastic/beats/metricbeat/module/logstash"
@@ -48,6 +50,7 @@ var (
 // MetricSet type defines all fields of the MetricSet
 type MetricSet struct {
 	*logstash.MetricSet
+	initialized sync.Once
 }
 
 // New create a new instance of the MetricSet
@@ -58,15 +61,18 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 	}
 
 	return &MetricSet{
-		ms,
+		MetricSet: ms,
 	}, nil
 }
 
 // Fetch methods implements the data gathering and data conversion to the right format
 // It returns the event which is then forward to the output. In case of an error, a
 // descriptive error must be returned.
 func (m *MetricSet) Fetch(r mb.ReporterV2) error {
-	err := m.init()
+	var err error
+	m.initialized.Do(func() {
+		err = m.init()
+	})
 	if err != nil {
 		if m.XPack {
 			m.Logger().Error(err)