chore: Correct documentation to include start.count metric (#125)

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SIGNAL-7436

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- [x] Code conforms to the [Elixir
Styleguide](https://github.com/christopheradams/elixir_style_guide)

## Problem

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I was looking at our telemetry on wms-service, and I realized that we
had a duplicate entry for `kafee.produce.stop`. I looked into it, and
noticed that the documentation had this problem as well and omitted
`start.count` despite mentioning it lines above. Furthermore, Datadog
has never caught the `kafee.produce.stop.count` metric. This leads me to
believe that this was incorrect and this should be `start` instead.

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I switched this to `counter` as well because that's what we typically
use, but let me know if that's off base.

lib/kafee/producer.ex

@@ -190,7 +190,7 @@ defmodule Kafee.Producer do
 
   The recommended collection of these metrics can be done via:
 
-      summary("kafee.produce.stop.count",
+      counter("kafee.produce.start.count",
         tags: [:topic, :partition]
       ),
       summary("kafee.produce.stop.duration",