[#3059] Fixed Phoenix 1.5 telemetry processing.

mix.exs

@@ -171,6 +171,8 @@ defmodule Pleroma.Mixfile do
        override: true},
       {:prometheus_plugs, "~> 1.1"},
       {:prometheus_phoenix, "~> 1.3"},
+      # Note: once `prometheus_phx` is integrated into `prometheus_phoenix`, remove the former:
+      {:prometheus_phx, github: "theblitzapp/prometheus-phx"},
       {:prometheus_ecto, "~> 1.4"},
       {:recon, "~> 2.5"},
       {:quack, "~> 0.1.1"},