Omit monitoring object from logstash_stats.logstash object (#16198)

* Omit monitoring object from logstash_stats.logstash object

* Removing unnecessary struct tag info

* Adding CHANGELOG entry

* Adding explanatory comment for nodeInfo

CHANGELOG.next.asciidoc

@@ -88,6 +88,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - 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]
 - Fix skipping protocol scheme by light modules. {pull}16205[pull]
+- Made `logstash-xpack` module once again have parity with internally-collected Logstash monitoring data. {pull}16198[16198]
 
 *Packetbeat*
 

metricbeat/module/logstash/node_stats/data_xpack.go

@@ -96,7 +96,14 @@ type nodeInfo struct {
 	Status      string   `json:"status"`
 	HTTPAddress string   `json:"http_address"`
 	Pipeline    pipeline `json:"pipeline"`
-	Monitoring  struct {
+}
+
+// inNodeInfo represents the Logstash node info to be parsed from the Logstash API
+// response. It contains nodeInfo (which is also used as-is elsewhere) + monitoring
+// information.
+type inNodeInfo struct {
+	nodeInfo
+	Monitoring struct {
 		ClusterID string `json:"cluster_uuid"`
 	} `json:"monitoring"`
 }
@@ -108,7 +115,7 @@ type reloads struct {
 
 // NodeStats represents the stats of a Logstash node
 type NodeStats struct {
-	nodeInfo
+	inNodeInfo
 	commonStats
 	Process   process                  `json:"process"`
 	OS        os                       `json:"os"`