Add ability to delete metrics

[fouadkada]

Preamble

This is still WIP, the functionality is only added for gauge metrics for the time being just to get an initial feel of whether this is a good approach or there is a better way to do this.

Problem statement

Benthos supports setting metrics using the metric processor. Once a metric is set it's only possible to change its value. Sometimes, and for certain use cases, it is necessary for a metric to be deleted, as in not exported anymore.

This PR aims to add a new configuration(delete_metric) for the metric processor that when it evaluates to true the metric should not be exported anymore.

For the time being, and to re-iterate, this is only specific for prometheus and for gauge just to test the waters. If there's a chance of this being adopted then I will add the functionality to the other metric types (counter, counter_by...)

[CLAassistant]

All committers have signed the CLA.