Add total message rates for RabbitMQ input plugin

[kerams]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

Can the rate values be calculated from the message counts at query time? If so this will be more flexible since you will be able to calculate over any unit of time such as messages/minute, as well as using less storage on the database.

We can definitely add messages_delivered_get.

[kerams]

No, I don't think so. The counts increase with every message and as far as I can tell they never reset (they certainly do not represent message counts for the sample period, which is 5s by default).

EDIT: I can now confirm that those are the total counts since the node's startup, which makes them far less interesting than the current rates.

[danielnelson]

Maybe you can use this query:

select non_negative_difference(last("messages_published")) as messages_published_rate from rabbitmq_overview where time > now() - 10m group by time(1m)

[kerams]

Oh, yeah, that didn't occur to me. However, I think it would be nice to have those rates as they are already recorded for every queue :). Your call.

[danielnelson]

I'd rather not have them, because they can't be aggregated and they are duplicate information. However, we should add the messages_delivered_get and we could add some documentation about this query.

[kerams]

Alright, I'll open a new PR for that