Database connection number monitoring

[sunzhaoyang]

Feature Request

Is your feature request related to a problem? Please describe:

#2127

Describe the feature you'd like:

1. Monitor the number of connections to upstream and downstream per worker
2. Exceeds set value (currently 1 connection?) Alarm

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[Ehco1996]

related to #2142

[Ehco1996]

maybe we can use this package ? https://github.com/dlmiddlecote/sqlstats

wow, prometheus_client_golang already Implemented this feature in prometheus/client_golang#866

cc @lance6716 @lichunzhu