Fixes in file prometheus-metrics-v1-to-v2.md based on changes to the Redis Enterprise (#1148)

* Fix typos in prometheus-metrics-v1-to-v2.md

- Replace `bdb` with `db` (the label value was renamed)
- Rename `endpoint_write_req` to `endpoint_write_requests`
- Rename `endpoint_conns` to `endpoint_client_connections`

* Simplify some queries

* Escaped `|`

* Fix some queries

* Update prometheus-metrics-v1-to-v2.md

Author: zviRosenfeldRedis