Pass-through metrics

[Sushisource]

My apps already output fully-formed metrics in Prometheus format straight onto stdout. I'd like to simply configure FluentD to route those metrics, unaltered, to this plugin so that they're exposed for scraping. Something like:

<filter message>
  @type prometheus
  <passthrough>    
    key foo
  </passthrough>
</filter>

Where "foo" contains metrics in the format specified here: https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md as well as here https://prometheus.io/docs/instrumenting/exposition_formats/

As far as I can tell, you can't do that right now. Is that a feature you'd consider adding? Thanks!