[components] Fixes Bugs
1497879,
1497892 - Add counter metric type to glean
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