Add link to conditional plug invocation

[krwenholz]

Change description

Minor docs update! I think Plug added this functionality late last year.

What problem does this solve?

Unplug is awesome, but I needed to conditionally check a few headers in ways that weren't obvious with that library. It's also nice to add one less library.

Example usage

  plug :metrics

  defp metrics(%Plug.Conn{} = conn, _opts) do
    bearer_string = "Bearer #{Application.get_env(:my_app, MyApp.PromEx)[:bearer_token]}"

    if conn.request_path == "/metrics" &&
         Plug.Conn.get_req_header(conn, "authorization")
         |> Enum.any?(fn i ->
           i == bearer_string
         end) do
      PromEx.Plug.call(conn, %{metrics_path: "/metrics", prom_ex_module: MyApp.PromEx})
    else
      conn
    end
  end

Checklist

• I have added unit tests to cover my changes.
• I have added documentation to cover my changes.
• My changes have passed unit tests and have been tested E2E in an example project.

[coveralls]

coverage: 76.931% (-0.2%) from 77.131%
when pulling 3f20f31 on krwenholz:patch-1
into 6f97943 on akoutmos:master.