Bump prometheus_exporter from 0.7.0 to 0.8.0

Bumps [prometheus_exporter](https://github.com/discourse/prometheus_exporter) from 0.7.0 to 0.8.0.
- [Release notes](https://github.com/discourse/prometheus_exporter/releases)
- [Changelog](https://github.com/discourse/prometheus_exporter/blob/master/CHANGELOG)
- [Commits](https://github.com/discourse/prometheus_exporter/compare/v0.7.0...v0.8.0)

---
updated-dependencies:
- dependency-name: prometheus_exporter
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -283,7 +283,7 @@ GEM
       ast (~> 2.4.1)
     path_expander (1.1.0)
     pg (1.2.3)
-    prometheus_exporter (0.7.0)
+    prometheus_exporter (0.8.0)
       webrick
     pry (0.13.1)
       coderay (~> 1.1)