The package provides everything you need.

So, given you use it in your code to declare and update metrics like this:

Prometheus.Metric.Boolean.declare([name: :my_metric, help: "My Metric", labels: [:label1, :label2], registry: :all])
Prometheus.Metric.Boolean.set([name: :my_metric, labels: ["foo", "bar"], registry: :all], true)

(I use registry: :all here because I don’t need the system metrics that are added by default by prometheus_ex)

You just need plug, or anything else that serves your metrics endpoint. The prometheus package gives you the metrics in the correct form. My code is simply this in a normal plug router.

  get "/metrics" do
    metrics = Prometheus.Format.Text.format(:all)
    send_resp(conn, 200, metrics)
  end

(Here I pass it the :all argument because I pushed my metrics to the :all registry)

That is only meant for things like queue workers where you want to send metrics of individual jobs.

That is what the prometheus_ex lib does.

Prometheus is an external tool, so you send events/metrics to prometheus. You can send metrics from different apps/projects, your metrics endpoint could retrieve data from prometheus to show it to a user? I think prometheus already has a UI/works with grafana so it might be better to just use those?