Output slow Ecto queries to logs

slow_query_handler.ex

defmodule MyApp.Telemetry do
  require Logger

  def handle_event([:my_app, :repo, :query], measurements, metadata, _config) do
    milliseconds = System.convert_time_unit(measurements.total_time, :native, :millisecond)

    # did the query take longer than 100ms?
    if milliseconds > 100 do
      # log it as a warning
      Logger.warn("SLOW QUERY: ms: #{milliseconds}, query: #{metadata.query}")
    end
  end
end

# in application.ex
:ok = :telemetry.attach("slow-query-handler", [:my_app, :repo, :query], &MyApp.Telemetry.handle_event/4, %{})