Ensure Pod UID is always present in pod metricset (elastic#12345)

Pod UID field is currently retrieved as part of the metadata enrichment
present for  all Kubernetes module metricsets. In case of errors or
missconfiguration, pod UID won't be present in resulting documents.

Since this field is available in the `pod` metricset, this change adds
it to Pod events, avoiding issues when enrichment fails.

CHANGELOG.next.asciidoc

@@ -239,6 +239,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Keep `etcd` followers members from reporting `leader` metricset events {pull}12004[12004]
 - Add overview dashboard to Consul module {pull}10665[10665]
 - New fields were added in the mysql/status metricset. {pull}12227[12227]
+- Always report Pod UID in the `pod` metricset. {pull}12345[12345]
 - Add Vsphere Virtual Machine operating system to `os` field in Vsphere virtualmachine module. {pull}12391[12391]
 - Add validation for elasticsearch and kibana modules' metricsets when xpack.enabled is set to true. {pull}12386[12386]
 

metricbeat/module/kubernetes/pod/data.go

@@ -65,6 +65,7 @@ func eventMapping(content []byte, perfMetrics *util.PerfMetricsCache) ([]common.
 				},
 			},
 			"name": pod.PodRef.Name,
+			"uid":  pod.PodRef.UID,
 
 			"cpu": common.MapStr{
 				"usage": common.MapStr{

metricbeat/module/kubernetes/pod/pod_test.go

@@ -51,6 +51,7 @@ func TestEventMapping(t *testing.T) {
 
 	testCases := map[string]interface{}{
 		"name": "nginx-deployment-2303442956-pcqfc",
+		"uid":  "beabc196-2456-11e7-a3ad-42010a840235",
 
 		"network.rx.bytes":  107056,
 		"network.rx.errors": 0,