Use Jason for rendering responses

Although Jason readme says Phoenix 1.4+ already does it by default, [it actually does it only for new projects](3bfb9f6e90/lib/phoenix.ex (L58-L59))

lib/pleroma/healthcheck.ex

@@ -0,0 +1,65 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Healthcheck do
+  @moduledoc """
+  Module collects metrics about app and assign healthy status.
+  """
+  alias Pleroma.Healthcheck
+  alias Pleroma.Repo
+
+  @derive Jason.Encoder
+  defstruct pool_size: 0,
+            active: 0,
+            idle: 0,
+            memory_used: 0,
+            healthy: true
+
+  @type t :: %__MODULE__{
+          pool_size: non_neg_integer(),
+          active: non_neg_integer(),
+          idle: non_neg_integer(),
+          memory_used: number(),
+          healthy: boolean()
+        }
+
+  @spec system_info() :: t()
+  def system_info do
+    %Healthcheck{
+      memory_used: Float.round(:erlang.memory(:total) / 1024 / 1024, 2)
+    }
+    |> assign_db_info()
+    |> check_health()
+  end
+
+  defp assign_db_info(healthcheck) do
+    database = Pleroma.Config.get([Repo, :database])
+
+    query =
+      "select state, count(pid) from pg_stat_activity where datname = '#{database}' group by state;"
+
+    result = Repo.query!(query)
+    pool_size = Pleroma.Config.get([Repo, :pool_size])
+
+    db_info =
+      Enum.reduce(result.rows, %{active: 0, idle: 0}, fn [state, cnt], states ->
+        if state == "active" do
+          Map.put(states, :active, states.active + cnt)
+        else
+          Map.put(states, :idle, states.idle + cnt)
+        end
+      end)
+      |> Map.put(:pool_size, pool_size)
+
+    Map.merge(healthcheck, db_info)
+  end
+
+  @spec check_health(Healthcheck.t()) :: Healthcheck.t()
+  def check_health(%{pool_size: pool_size, active: active} = check)
+      when active >= pool_size do
+    %{check | healthy: false}
+  end
+
+  def check_health(check), do: check
+end