Is your feature request related to a problem? Please describe.
The logs coming through fluentforward receiver can have pod_uid associated with them and no information about the IP address of the pod. This could be easily leveraged by adding capability to associate pod with the record using either IP address (as it is now) or Pod UID.

Describe the solution you'd like
Let the Kubernetes Processor cache metadata tags using Pod UID as key (similarly as it's done for IP - two maps could be used)

The name of the tag containing Pod UID can be defaulted to pod_uid and optionally made configurable, e.g. a new configuration entry might be added:

pod_uid_matching_labels - by default, [pod_uid]; if empty - no matching by Pod uid applied

Alternatively, taking into account #1079 and #1078, maybe a more streamlined configuration approach could be applied, e.g.

pod_association:
  ip:
    labels: [ip, k8s.pod.ip, host.hostname]
    connection_source_extraction_enabled: true
  uid:
    labels: [pod_uid]

Additional context

I have prepared a document that describes this in more context: https://docs.google.com/document/d/1QlFbXz0eQUaKXK1WrnEs3VqRkcPD2RAdXJdJMjnOSd8/edit#