Promtail configuration with Environment Variable Interpolation does not appear to be working

So I’m trying to use an environment variable for configuration of promtail clients per the documentation at the grafana documentation

  - job_name: varlogs
  - role_arn: ${INSTANCE_PROFILE_ARN}

Here, I’m trying to set the INSTANCE_PROFILE_ARN for EC2 discovery so that we can accomplish interactions via the instance profile rather than Secret / Key etc. Promtail will start and scrape, but it fails accessing AWS:

is not authorized to perform: sts:AssumeRole on resource: ${INSTANCE_PROFILE_ARN}

I thought it might have been caused by not having a default, but if I put anything in that structure as a default (ie ${INSTANCE_PROFILE_ARN:123}), it just prints that raw text.

My gut feeling is that because these are basically part of the prometheus definitions for scrape configs, variable interpolation isn’t happening.

what version do you have?