* Factorise metrics code for KeyValueStorage database

- introduce `MonitorableKeyValueStorage`
- factorise code
- remove metrics instanciation in `RocksDbKeyValueStorage` and `ColumnarRocksDbKeyValueStorage`

* Rename class

* refactor Enclave

- use final modifiers when appropriate to comply with Pantheon global policy
- use `assertj` assertions in tests
- don't use local variables when not necessary
- check exception message for `upCheck` method