Update metric name construction for app workers

Currently, converstation metrics are constructed in the form `<prefix>.<account_key>.<conversation_key>.<metric_name>.<agg_method>`, and account metrics in the form  `<prefix>.<account_key>.<store_name>.<metric_name>.<agg_method>`. The plan is to change these to  `<prefix>.<account_key>.conversations.<conversation_key>.<metric_name>.<agg_method>` and `<prefix>.<account_key>.stores.<store_name>.<metric_name>.<agg_method>` respectively.