Sometimes we need to manually add metrics in order to calculate the value for other metrics. E.g:

• __internal_visits is fetched when querying bounce rate, visit duration, or views_per_visit, as it is needed to calculate these from imported data.

• visitors metric might be added manually via maybe_add_visitors_metric/1, in order to be able to calculate conversion rate.

This function can be used for stripping those metrics from a breakdown (list), or an aggregate (map) result. We do not want to return metrics that we're not requested.

This function adds the visitors metric into the list of given metrics if it's not already there and if it is needed for any of the other metrics to be calculated.