Lewis

@scott-klaytn Could you review this PR? Thanks.

> I added PebbleDB unit test into `db_manager_test.go`. and the tests I changed run successfully on my local machine. However, Im not sure why the CI jobs failed. (with below...

> I added metrics and all the logic from [pebble.go](https://github.com/ethereum/go-ethereum/blob/master/ethdb/pebble/pebble.go#L455) in Geth > > Do I need to make the metrics key the same as in the [leveldb_database.go](https://github.com/klaytn/klaytn/blob/dev/storage/database/leveldb_database.go#L358) code? They...