Fix RabbitMQ regression in #9383

[srebhan]

Required for all PRs:

• Updated associated README.md.
• Wrote appropriate unit tests.

resolves #9383

This resolves the wrong types for _avg metrics. Those errors went unnoticed before checking the return types (#8992) and returned invalid values (all zero) if the server responded with as floating-point formatted number by chance.
This PR furthermore tries to handle missing plugins (e.g. federation) more gracefully providing the server sent error to the user. To avoid those errors altogether, a (backward-compatible) metric-filter is introduced such that the user can disable erroneous metrics.

[akrantz01]

Overall LGTM. I just left some comments on some of the type conversion for Memory.Total.

[telegraf-tiger]

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[srebhan]

@akrantz01 thanks for review. I changed the code according to your suggestions except for the map[string]interface{} part, which does not work otherwise.