Allow icinga checks to write prometheus metrics files

The prometheus data are readable by everyone so adding icinga in the prometheus groups seems to not open any hole

Related to T3129

Test Plan

• pergamon (collected resources not visible)

diff origin/production/pergamon.softwareheritage.org current/pergamon.softwareheritage.org
*******************************************
+ File[/var/lib/prometheus/node-exporter] =>
   parameters =>
     "ensure": "directory",
     "group": "prometheus",
     "mode": "0775",
     "owner": "prometheus"
*******************************************
*** End octocatalog-diff on pergamon.softwareheritage.org

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Migrated from D7844 (view on Phabricator)