Log when we're calculating statistics or formatting

Helps see which step went wrong and at what point we are right
now.

So, if say memory blows up or something takes super long it's
easier to discern whether that is a benchmark currently running,
the statistics calculation or the formatter. #389

lib/benchee.ex

@@ -69,6 +69,8 @@ for {module, moduledoc} <- [{Benchee, elixir_doc}, {:benchee, erlang_doc}] do
     @doc false
     defdelegate collect(suite, printer), to: Benchee.Benchmark
     defdelegate statistics(suite), to: Benchee.Statistics
+    @doc false
+    defdelegate statistics(suite, printer), to: Benchee.Statistics
     defdelegate relative_statistics(suite), to: Benchee.RelativeStatistics
     defdelegate load(suite), to: Benchee.ScenarioLoader
     defdelegate profile(suite), to: Benchee.Profile

lib/benchee/formatter.ex

@@ -11,6 +11,7 @@ defmodule Benchee.Formatter do
   through `output/3`.
   """
 
+  alias Benchee.Output.ProgressPrinter
   alias Benchee.{Suite, Utility.Parallel}
   alias Benchee.Utility.DeepConvert
 
@@ -50,6 +51,8 @@ defmodule Benchee.Formatter do
   """
   @spec output(Suite.t()) :: Suite.t()
   def output(suite = %{configuration: %{formatters: formatters}}) do
+    print_formatting(suite.configuration)
+
     {parallelizable, serial} =
       formatters
       |> Enum.map(&normalize_module_configuration/1)
@@ -64,6 +67,15 @@ defmodule Benchee.Formatter do
     suite
   end
 
+  # This is a bit of a hack, but we can't DI the printer like we usually do as we made the _great_
+  # decisions to use both `output/1` and `output/2` as part of the public interface. Adding the
+  # printer to `output/1` would clash with `output/2` and its second parameter is _also_ a module.
+  # So, we're abusing the `assigns` in the config a bit.
+  defp print_formatting(config) do
+    printer = config.assigns[:test][:progress_printer] || ProgressPrinter
+    printer.formatting(config)
+  end
+
   defp normalize_module_configuration(formatter) when is_function(formatter, 1), do: formatter
 
   defp normalize_module_configuration({module, opts}) do

lib/benchee/output/progress_printer.ex

@@ -0,0 +1,15 @@
+defmodule Benchee.Output.ProgressPrinter do
+  @moduledoc false
+
+  def calculating_statistics(%{print: %{benchmarking: false}}), do: nil
+
+  def calculating_statistics(_config) do
+    IO.puts("Calculating statistics...")
+  end
+
+  def formatting(%{print: %{benchmarking: false}}), do: nil
+
+  def formatting(_config) do
+    IO.puts("Formatting results..")
+  end
+end

lib/benchee/statistics.ex

@@ -7,6 +7,7 @@ defmodule Benchee.Statistics do
   """
 
   alias Benchee.{CollectionData, Conversion.Duration, Scenario, Suite, Utility.Parallel}
+  alias Benchee.Output.ProgressPrinter
 
   require Integer
 
@@ -115,7 +116,7 @@ defmodule Benchee.Statistics do
       ...>   }
       ...> ]
       iex> suite = %Benchee.Suite{scenarios: scenarios}
-      iex> statistics(suite)
+      iex> statistics(suite, Benchee.Test.FakeProgressPrinter)
       %Benchee.Suite{
         scenarios: [
           %Benchee.Scenario{
@@ -161,7 +162,9 @@ defmodule Benchee.Statistics do
 
   """
   @spec statistics(Suite.t()) :: Suite.t()
-  def statistics(suite) do
+  def statistics(suite, printer \\ ProgressPrinter) do
+    printer.calculating_statistics(suite.configuration)
+
     percentiles = suite.configuration.percentiles
 
     update_in(suite.scenarios, fn scenarios ->