Should we discourage array-based attributes on metrics?

[trask]

We've hesitated to add array-based attributes (e.g. db.collection.names) in the past due to concern that they aren't well supported today in metrics backends.

Another recent example is db.cosmosdb.regions_contacted.

Capturing these as attributes on metrics gives users the chance to aggregate across metrics where db.collection.names contains X, or db.cosmosdb.regions_contacted contains Y, which seems to be useful.

Another benefit of including these attributes on metrics is that they can be meaningful characteristics to split time series on.