Adds configurable 'extra' options (pinterest#57)

* adds extra options

* change extra to subscribe_options, add tests

* cleanup: more idiomatic test case

README.md

@@ -99,3 +99,11 @@ By default, Elixometer only includes the `exometer_core` package. However, some
     ]
   end
 ```
+
+In case a reporter allows for extra configuration options on subscribe, you can configure them in your `elixometer` config like so:
+
+```elixir
+       config(:elixometer,
+       	    ...
+       	    subscribe_options: [{:tag, :value1}])
+```

lib/elixometer.ex

@@ -345,11 +345,12 @@ defmodule Elixometer do
       cfg = Application.get_all_env(:elixometer)
       reporter = cfg[:reporter]
       interval = cfg[:update_frequency]
+      subscribe_options = cfg[:subscribe_options] || true
 
       if reporter do
         :exometer.info(metric_name)
         |> Keyword.get(:datapoints)
-        |> Enum.map(&(:exometer_report.subscribe(reporter, metric_name, &1, interval)))
+        |> Enum.map(&(:exometer_report.subscribe(reporter, metric_name, &1, interval, subscribe_options)))
       end
       :ets.insert(@elixometer_table, {{:subscriptions, metric_name}, true})
     end

test/elixometer_test.exs

@@ -281,4 +281,12 @@ defmodule ElixometerTest do
     assert {:error, :not_found} == get_metric_value("elixometer.test.bad.bad")
   end
 
+  test "a subscription that has additional subscription options" do
+    Application.put_env(:elixometer, :subscribe_options, [some_option: 42])
+
+    update_counter("subscribe_options", 1)
+
+    assert subscription_exists "elixometer.test.counters.subscribe_options"
+    assert [some_option: 42] = Reporter.options_for("elixometer.test.counters.subscribe_options")
+  end
 end

test/support/test_reporter.ex

@@ -35,13 +35,38 @@ defmodule Elixometer.TestReporter do
 
   def value_for(metric_name, datapoint) when is_bitstring(metric_name) do
     metric_name
-    |> String.split(".")
-    |> Enum.map(&String.to_atom/1)
+    |> to_elixometer_name
     |> value_for(datapoint)
   end
 
   def value_for(metric_name, datapoint) do
     {:ok, value} = :exometer.get_value(metric_name, datapoint)
     value
   end
+
+  def options_for(metric_name) when is_bitstring(metric_name) do
+    metric_name
+    |> to_elixometer_name
+    |> options_for
+  end
+
+  def options_for(metric_name) do
+    Application.get_env(:elixometer, :reporter)
+    |> :exometer_report.list_subscriptions
+    |> Enum.filter_map(
+      fn {name, _, _, _}  -> name == metric_name end,
+      fn {_, _, _, extra} -> extra end)
+    |> case do
+      [hd | _] -> hd
+      _        -> nil
+    end
+  end
+
+  # Helpers
+
+  defp to_elixometer_name(metric_name) do
+    metric_name
+    |> String.split(".")
+    |> Enum.map(&String.to_atom/1)
+  end
 end