New metric metadata: metric aggregation

[diosmosis]

Summary

Refs matomo-org/looker-studio-connector#3
Refs matomo-org/looker-studio-connector#29

In order for Looker Studio to further group data in reports after they are requested from Matomo, it needs to know how metrics should be aggregated. For example, nb_visits should be sum-ed and max_actions should be max-ed, etc.

Similar to <metricTypes>, having individual metric aggregation information in the Metadata API output would allow the connector to forward this information to Looker Studio.

Example API output:

<metadata>
    ...
    <metricAggregation>
        <nb_visits>sum</nb_visits>
        <max_actions>max</max_actions>
        <nb_uniq_visitors>none</nb_uniq_visitors>
    </metricAggregation>
</metadata>