Fix deliver_get in RabbitMQ

[kerams]

The json output has apparently never been parsed correctly.

Required for all PRs:

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

[danielnelson]

I think this value is reported in rabbitmq_overview as messages_delivered and in rabbitmq_queue as messages_deliver_get. Can you explain what this change does?

[kerams]

These are 2 separate values (https://pulse.mozilla.org/doc/stats.html). In #3596 I exposed messages_deliver_get into the rabbitmq_overview measurement, but it turns out it's always reported as 0 (in rabbitmq_queue too) because of the bug that I am fixing in this PR.

[kerams]

In other words, both messages_delivered and messages_delivered_get should be available in rabbitmq_overview as well as rabbitmq_queue, but the latter is currently always 0.

[danielnelson]

I see, because of the underscore we need the struct tag.