Support average values for event data of type number

[Tobi696]

Describe the feature or enhancement

We have a search engine for hotels and use umami for tracking how long it takes users to see the first search result. It would be very nice to have the average for a numeric event data field for such cases.

[github-actions]

This issue is stale because it has been open for 60 days with no activity.

[github-actions]

This issue was closed because it has been inactive for 7 days since being marked as stale.