Add support for `web_exporter:`

This adds a possibility to configure gitlab!17943 (merged) and gitlab!17962 (merged), the:

gitlab:
  monitoring:
    web_exporter:
    #  enabled: true
    #  address: localhost
    #  port: 8083
    #  blackout_seconds: 10

Related to: gitlab#30037 (closed) gitlab#30201 (closed)