Parse Error preventing environment specific metric from working

Summary

Unable to create custom metric in Prometheus Integration

Steps to reproduce

Following the instructions provided here: https://ops.gitlab.net/help/user/project/integrations/prometheus#adding-additional-metrics-premium

I'm using the following query: sum(irate(registry_http_requests_total{environment=%{ci_environment_slug}}[1m])) by (handler, code)

The response during validation is hidden from view. It can be found in Dev Tools, however:

{
  "success": true,
  "last_update": "2019-09-25T17:17:02.147Z",
  "query": {
    "valid": false,
    "error": "parse error at char 52: unexpected character inside braces: '%'"
  }
}

What is the expected correct behavior?

When following the documentation, I would've expected a working query.

Relevant logs and/or screenshots

Output of checks

GitLab.com