feat: expose system metrics (#4)

Use prom_ex to expose system metrics to prometheus and create dashboards.

Co-authored-by: Joao P Dubas <joao.dubas@gmail.com>
Reviewed-on: #4

lib/wabanex_web/endpoint.ex

@@ -16,6 +16,8 @@ defmodule WabanexWeb.Endpoint do
 
   socket "/live", Phoenix.LiveView.Socket, websocket: [connect_info: [session: @session_options]]
 
+  plug PromEx.Plug, prom_ex_module: Wabanex.PromEx
+
   # Serve at "/" the static files from "priv/static" directory.
   #
   # You should set gzip to true if you are running phx.digest

lib/wabanex_web/schema/types/custom/date_range.ex

@@ -17,10 +17,11 @@ defmodule WabanexWeb.Schema.Types.Custom.DateRange do
   end
 
   @spec serialize_range(PgRanges.DateRange.t()) :: String.t()
-  @spec serialize_range(list(DateTime.t() | nil)) :: String.t()
+  @spec serialize_range(list(DateTime.t() | nil | atom)) :: String.t()
   defp serialize_range(%PgRanges.DateRange{lower: start_range, upper: end_range}),
     do: serialize_range([start_range, end_range])
 
+  defp serialize_range([start_range, :unbound]), do: serialize_range([start_range, nil])
   defp serialize_range([start_range, nil]), do: "#{Date.to_iso8601(start_range)},"
 
   defp serialize_range([start_range, end_range]),