prometheus: Well categorize the firewall metrics

Related to T2755

Test Plan

octocatalog-diff pergamon:

diff origin/production/pergamon.softwareheritage.org current/pergamon.softwareheritage.org
*******************************************
+ Profile::Prometheus::Export_scrape_config[firewall_glyptotek.internal.softwareheritage.org] =>
   parameters =>
     "job": "firewall",
     "labels": {
       "instance": "glyptotek.internal.softwareheritage.org"
     },
     "metrics_path": "/metrics",
     "scheme": "http",
     "target": "glyptotek.internal.softwareheritage.org:9100"
*******************************************
+ Profile::Prometheus::Export_scrape_config[firewall_pushkin.internal.softwareheritage.org] =>
   parameters =>
     "job": "firewall",
     "labels": {
       "instance": "pushkin.internal.softwareheritage.org"
     },
     "metrics_path": "/metrics",
     "scheme": "http",
     "target": "pushkin.internal.softwareheritage.org:9100"
*******************************************
- Profile::Prometheus::Export_scrape_config[glyptotek.internal.softwareheritage.org]
*******************************************
- Profile::Prometheus::Export_scrape_config[pushkin.internal.softwareheritage.org]
*******************************************
*** End octocatalog-diff on pergamon.softwareheritage.org

---

Migrated from D4484 (view on Phabricator)