Add Prometheus settings

The following settings are in the backend config/description.exs:

  %{
    group: :prometheus,
    key: Pleroma.Web.Endpoint.MetricsExporter,
    type: :group,
    description: "Prometheus app metrics endpoint configuration",
    children: [
      %{
        key: :enabled,
        type: :boolean,
        description: "[Pleroma extension] Enables app metrics endpoint."
      },
      %{
        key: :ip_whitelist,
        type: [{:list, :string}, {:list, :charlist}, {:list, :tuple}],
        description:
          "[Pleroma extension] If non-empty, restricts access to app metrics endpoint to specified IP addresses."
      },
      %{
        key: :auth,
        type: [:boolean, :tuple],
        description: "Enables HTTP Basic Auth for app metrics endpoint.",
        suggestion: [false, {:basic, "myusername", "mypassword"}]
      },
      %{
        key: :path,
        type: :string,
        description: "App metrics endpoint URI path.",
        suggestions: ["/api/pleroma/app_metrics"]
      },
      %{
        key: :format,
        type: :atom,
        description: "App metrics endpoint output format.",
        suggestions: [:text, :protobuf]
      }
    ]
  }