[k8sattributes] `key_regex` should work even if `tag_name` is not present

[ChrsMark]

ref:

opentelemetry-collector-contrib/processor/k8sattributesprocessor/config.go

Lines 201 to 203 in 766b001

	// KeyRegex is a regular expression used to extract a Key that matches the regex.
	// Out of Key or KeyRegex, only one option is expected to be configured at a time.
	KeyRegex string `mapstructure:"key_regex"`

In the docstring it is mentioned the following:

// When not specified, a default tag name will be used of the format:
//   - k8s.pod.annotations.<annotation key>  (or k8s.pod.annotation.<annotation key> when processor.k8sattributes.EmitV1K8sConventions is enabled)
//   - k8s.pod.labels.<label key>  (or k8s.pod.label.<label key> when processor.k8sattributes.EmitV1K8sConventions is enabled)

This means that the following works as expected:

labels:
  - key: app
    from: pod

and extracts the label as k8s.pod.labels.app: nginx

We should support the same for the key_regex option too:

        labels:
          - key_regex: (.*)
            from: pod

The above does not seem to work as expected: it should catch all labels and extract them using k8s.pod.labels.<label key> pattern)

[github-actions]

Pinging code owners for processor/k8sattributes: @dmitryax @fatsheep9146 @TylerHelmuth @ChrsMark @odubajDT. See Adding Labels via Comments if you do not have permissions to add labels yourself. For example, comment '/label priority:p2 -needs-triage' to set the priority and remove the needs-triage label.

[iblancasa]

Can I take it?