add description fields to granfana metrics

[lysu]

Feature Request

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

Current there are too many metrics and most of our customer often ask the question "what's this metric mean?"

for example:...

Describe the feature you'd like:

maybe we should add more information in grafana's description part

then user can see metric detail from

's pop up.

Describe alternatives you've considered:

we should add it in English or native language?

Teachability, Documentation, Adoption, Migration Strategy:

[zz-jason]

actually, we have documents about each metrics, see this: https://pingcap.com/docs/dev/reference/key-monitoring-metrics/tidb/