Record accessor in Labels does not work on all fields

[idanya]

I am trying to export a metric with labels created by kubernetes_metadata filter. I'm using the following match:

<filter nginx.error.upstream.timeout>
  @type parser
  key_name log
  <parse>
    @type regexp
    expression upstream: \"http://(?<upstream>.*?)/.*?\" 
  </parse>
</filter>

<match nginx.error.upstream.*>
    @type prometheus        
    <metric>
      name nginx_upstream_error
      type counter
      desc The total number of Nginx upstream timeouts.    
      <labels>
        error ${tag_parts[3]}                              
        upstream $.upstream
        origin-pod $.kubernetes.pod_name
        origin-namespace $.kubernetes.namespace_name
      </labels>
    </metric>
</match>

Output in /metrics looks like this:

# TYPE nginx_upstream_error counter
# HELP nginx_upstream_error The total number of Nginx upstream timeouts.
nginx_upstream_error{error="timeout",upstream="<ip>:<port>",origin-pod="",origin-namespace=""} 3.0

For some reason $.kubernetes.pod_name and $.kubernetes.namespace_name doesn't work and stay empty. The source of this tag goes through the kubernetes metadata plugin:

<filter kubernetes.**>
  @type kubernetes_metadata  
</filter>

I've validated that the object kubernetes with the specified fields is there (my whole logging infrastructure is based on it). What is even stranger is that the upstream label works! Seems like the record accessor does not work on fields came out from k8s enrichment but does work on fields came out of the previous parser.