Refact grafana json files for classis and next gen

## Enhancement
In https://github.com/pingcap/tidb/pull/61688, we added next gen specific grafana json files to distinguish metrics with keyspace_name labels and not. However, it shares many common code with classic ones, and introduces double-work for modifying exsiting metrics. Even worse, deveplopers may forget to update the other one. It would be helpful to use jsonnet script to refact them, and reuse code as much as possible.
Before this feature is completed, we may need to review all metrics related changes.