Adds CountersStorageEngine and CounterMetricType to glean.  Updated StorageEngineManager, adding 'counter' to default storage engines.
Changed CounterMetricType to read from storage engine

Rather than using a local variable to track the current count, as this could get out of sync if app  pushed to background.  Also fixed some copypasta errors, changing "String" to "Counter".
Changes to make use of combinator and async engine


Changes per Alessio


Updated String and UUID metric type classes to internal access


Update message


Increase code coverage