Collect: set env variable into metadata

Question

I'm writing a collectd config for my application, is it possible to set an env variable as the metadata for my metrics? for example:

<Chain "my_chain">
  <Rule "my_rule">
    ...
    <Target "set">
      MetaData "foo" "${MY_ENV_VAR}"
    </Target>
    ...
  </Rule>
</Chain>