Include kube_statefulset_status_replicas_ready metric in kubernetes Metricbeat module

[eedugon]

Describe the enhancement:

As part of the state_statefulset metricseat some users are reporting the need of using the metric kube_statefulset_status_replicas_ready, which is not collected at the moment, but it's returned by kube-state-metrics:

https://github.com/elastic/beats/blob/master/metricbeat/module/kubernetes/state_statefulset/state_statefulset.go

That metric is useful to build better dashboards for kubernetes monitoring.

My recommendation is also to review the available metrics to see if there are other similar metrics that we might be discarding at the moment.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)