application.ex

defmodule Friends.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false

  use Application

  def start(_type, _args) do
    # List all child processes to be supervised
    children = [
      Friends.Repo,
      {Friends.Instrumentation, []}
      # Starts a worker by calling: Friends.Worker.start_link(arg)
      # {Friends.Worker, arg},
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: Friends.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

instrumentation.ex

defmodule Friends.Instrumentation do
  require Logger

  @event_subscriptions [
    [:friends, :repo, :query]
  ]
  @handler_id "instrumentation"
  @threshold 10

  @doc false
  def start_link do
    :ok = :telemetry.attach_many(@handler_id, @event_subscriptions, &handle_event/4, [])
    :ignore
  end

  def child_spec(_opts) do
    %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      type: :worker,
      restart: :permanent,
      shutdown: 500
    }
  end

  defp handle_event([:friends, :repo, :query], total_time, metadata, _config) do
    if to_milliseconds(total_time) >= @threshold do
      %{query_time: query_time} = metadata

      Logger.warn("query_time=#{to_milliseconds(query_time)}")
    end
  end

  ## Helpers

  defp to_milliseconds(time_native) do
    System.convert_time_unit(time_native, :native, :milliseconds)
  end
end

people_table.txt

friends_repo=# select * from people;
 id | first_name | last_name | age
----+------------+-----------+-----
  1 | Alice      | Cooper    |  70
  2 | Bob        | Marley    |  10
  3 | Charlie    | Sheen     |  55
(3 rows)