Do not expose info metric for nil objects

[chrischdi]

What this PR does / why we need it:

Small follow-up to #1777 .

The current code exposes info metrics independent of the value of the object referenced by path.
With this change, it won't expose an info metric, if the object's value at path is nil. If path is not set, it will always expose a metric because we at least have metadata at the object.

Use case is the following:

In Cluster API there is an annotation which is used to pause objects (cluster.x-k8s.io/paused). The value of the annotation could be the empty string.

A use case is to have a metric to see if objects are paused (cluster.x-k8s.io/paused label exists, independent of the value, which includes the empty string "" as value).

So the challenge here is to determine if the label exists or not.
Currently we could expose the label as label on the metric, but the empty string would be semantically equal to the label not being set at all when querying using prometheus.

An example:

Consider the following objects:

apiVersion: cluster.x-k8s.io/v1beta1
kind: Cluster
metadata:
  name: paused-1
  labels:
    cluster.x-k8s.io/paused: ""
---
apiVersion: cluster.x-k8s.io/v1beta1
kind: Cluster
metadata:
  name: paused-2
  labels:
    cluster.x-k8s.io/paused: "foo"
---
apiVersion: cluster.x-k8s.io/v1beta1
kind: Cluster
metadata:
  name: not-paused
  labels: {}

And the following CR configuration:

kind: CustomResourceStateMetrics
spec:
  resources:
  - groupVersionKind:
      group: cluster.x-k8s.io
      kind: Cluster
      version: v1beta1
    metricNamePrefix: capi_cluster
    labelsFromPath:
      name: [metadata, name]
      namespace: [metadata, namespace]
      uid: [metadata, uid]
    metrics:
    - each:
        info:
          path:
          - metadata
          - annotations
          - cluster.x-k8s.io/paused
          labelsFromPath:
            value: []
        type: Info
      name: annotation_paused

Without this change, the created metrics would be as follows (I removed the namespace and uid labels):

capi_cluster_annotation_paused{name="paused-1",value=""} 1
capi_cluster_annotation_paused{name="paused-2",value="foo"} 1
capi_cluster_annotation_paused{name="not-paused"} 1

PromQL queries will result in the following, which makes it impossible to see if paused-1 is really paused:

capi_cluster_annotation_paused{name="paused-1"} 1
capi_cluster_annotation_paused{name="paused-2",value="foo"} 1
capi_cluster_annotation_paused{name="not-paused"} 1

With this change, the created metrics wqould be as follows (again, I removed the namespace and uid labels):

capi_cluster_annotation_paused{name="paused-1",value=""} 1
capi_cluster_annotation_paused{name="paused-2",value="foo"} 1

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

May decrease number of metrics for CR configured Info metrics which define a path.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[mrueg]

/lgtm

thanks for the follow-up PR!

[k8s-ci-robot]

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