Bump prometheus.version in /modules/metrics-exporter

Bumps `prometheus.version` from 0.11.0 to 0.12.0.

Updates `simpleclient` from 0.11.0 to 0.12.0
- [Release notes](https://github.com/prometheus/client_java/releases)
- [Commits](prometheus/client_java@parent-0.11.0...parent-0.12.0)

Updates `simpleclient_common` from 0.11.0 to 0.12.0
- [Release notes](https://github.com/prometheus/client_java/releases)
- [Commits](prometheus/client_java@parent-0.11.0...parent-0.12.0)

Updates `simpleclient_hotspot` from 0.11.0 to 0.12.0
- [Release notes](https://github.com/prometheus/client_java/releases)
- [Commits](prometheus/client_java@parent-0.11.0...parent-0.12.0)

---
updated-dependencies:
- dependency-name: io.prometheus:simpleclient
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: io.prometheus:simpleclient_common
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: io.prometheus:simpleclient_hotspot
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

modules/metrics-exporter/pom.xml

@@ -13,7 +13,7 @@
   <properties>
     <opencast.basedir>${project.basedir}/../..</opencast.basedir>
     <checkstyle.skip>false</checkstyle.skip>
-    <prometheus.version>0.11.0</prometheus.version>
+    <prometheus.version>0.12.0</prometheus.version>
   </properties>
   <dependencies>
     <!-- Opencast dependencies -->