As we gradually move from statsd to prometheus, we change the metric to
be a prometheus metric rather than statsd.
The change worth pointing out is that we have dropped the 'successful'
and 'failed' statuses from the metrics. I don't think it's useful to
have these statuses. It's very rare for an inbound message to fail when
we receive it and when it does, we raise an error and see it in our
logs. We aren't going to be looking at a graph of it as it's a rare
event, not typical behaviour that we want to monitor with a graph.
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