Add support to labeled_custom_distribution (Glean)

[edugfilho]

Requested on Slack:

janv

Hi, would it be possible to add support for labeled_custom_distribution to glam ? We recently added a new metric using this special type, here's a link https://glam.telemetry.mozilla.org/fog/probe/quotamanager_initialize_repository_number_of_iterations/explore?aggType=avg&ping_type=*

It looks like this type is similar to keyed_histograms on legacy telemetry