pkg/metrics: add query duration metric by resource group

## Enhancement

When `record-db-label = true`, `tidb_server_handle_query_duration_seconds` includes the `db` label. In clusters with many databases, this can create high metric cardinality.

Some alerting rules only need query duration split by `resource_group`. They do not need the `db` or `sql_type` dimensions, and they do not need a Grafana dashboard panel.

Add a query duration metric that keeps only the `resource_group` label, so alerting can avoid the extra cardinality from `db` and `sql_type`.