Fix stats db metrics

[elmattic]

Issue summary

Typing http://localhost:6116/stats/db in a browser when Forest + paritydb backend is running results in:

# RocksDB statistics:
Not enabled. Set enable_statistics to true in config and restart daemon

We would like proper comment and help message to enable db statistics. Ie:

# ParityDb statistics:
Not enabled. Set stats to true in parity_db section and restart daemon

But even when forcing stats to true in default ParityDbConfig constructor shows nothing.

Acceptance Criteria

• Proper metrics comment and help message is shown when ParityDb backend is used
• ParityDb stats are correctly displayed
• Make sure everything still works for RocksDB backend

Other information and links

forest/utils/metrics/src/lib.rs

Line 71 in a1f2567

async fn collect_db_metrics<DB>(