Forward-friendly exporter plug

[chulkilee]

The current exporter plug is serving metrics on given path (/metrics by default), or passing the conn. This is for plug pipeline... and it doesn't work for phoenix router!

  # phoenix example
  # in endpoint

  plug My.PrometheusExporter.

This works... but it sounds strange. For example, to change the path, I need to set it at application config of the exporter module, not when using it.

One option is to add plug option :path.. but that is wrong approach, since plug can be composable!

Instead of we make this as "app" like plug having lots of configs... we can just have very simple exporter plug use forward mechanism. (Plug - forward/2 / Phoenix - forward/4).

  # phoenix example

  pipeline :default do
    plug :accepts, ["json"]
  end

  scope "/" do
    pipe_through :default
    forward "/metrics", My.PrometheusExporter
  end

In this case, path for metrics is configured at router, not on application config of the exporter module. Also the exporter plug runs only on that path (not for all requests as in pipeline)

[deadtrickster]

I like that! what needs to be done?

[svileng]

Here's how the bamboo library does it, they have an EmailPreviewPlug plug that serves a page for viewing sent emails:

https://github.com/thoughtbot/bamboo/blob/master/lib/bamboo/plug/sent_email_viewer/email_preview_plug.ex