Associate K8s metrics with resource attributes

[ChrsMark]

At the moment most of the K8s metrics do not define attributes.

We should have recommendations/requirements on what resources metrics can/must associate with, e.g. k8s.pod, k8s.node etc.

Originally posted by @jsuereth in #1406 (review)

[lmolkova]

Related: #1276

There is no way to do it in yaml yet and we definitely need it, but it should not block us from documenting resources in markdown as a first step.