While there are plugins for MongoDB and RabbitMQ (several for each technology, even!), New Relic did not write any of them and we cannot offer any official recommendations or advice on their usage. It appears that at least one of each has source available on github, so in the event that you try them out and find that they won't alert on your desired metric, then you should be able to fork those codebases and change the GUID, publish a new version of the plugin, and change the summary metrics to include the one(s) you are interested in. Since I don't have an example mongo or rabbitmq instance spun up, I can't verify which metrics are alertable already, but the reason for my suggestion is that any plugin can alert on a max of 5 metrics as documented here: https://docs.newrelic.com/docs/plugins/developing-plugins/structuring-your-plugin/creating-summary-metrics-plugins#component_alerts - so if the published versions don't alert on what is most important to you, this is relatively easy to overcome.
You can of course ask the plugin authors (contact locations are linked from the info pages on their various plugins, as you click through from our Plugin Central directory) to consider making the change for you, since they are in charge of supporting their plugins - but note that this is not something they are paid for; they donated their time to the community to write plugins - so they are not necessarily obligated to help you.