Ensure Pod UID is always present in pod metricset

[exekias]

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.

[kaiyan-sheng]

Do we need to update metricbeat/module/kubernetes/pod/_meta/data.json as well?

[fearful-symmetry]

Forgive me if I'm a little behind on the k8s metricsets, is there a reason uid isn't in fields.yml? I don't see it.

[exekias]

Forgive me if I'm a little behind on the k8s metricsets, is there a reason uid isn't in fields.yml? I don't see it.

This is a good point, happy to explain: add_kubernetes_metadata processor already configures some of the fields that this module reports. We avoid double-registering them, as that could cause problems:

beats/libbeat/processors/add_kubernetes_metadata/_meta/fields.yml

Line 16 in 3af1f29

- name: pod.uid

. But the actual result is that this field was already registered.

Do we need to update metricbeat/module/kubernetes/pod/_meta/data.json as well?

This is mostly defensive code, as this module was already reporting this field through add_metadata enrichment, that's why current data.json already has the field:

beats/metricbeat/module/kubernetes/pod/_meta/data.json

Line 15 in 3af1f29

"uid": "b89a812e-18cd-11e9-b333-080027190d51",

[kaiyan-sheng]

Good to know! Thanks for answering. Besides the merge conflict, it looks good.