Graceful upgrade of TiDB

[DanielZhangQD]

Feature Request

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

https://docs.pingcap.com/tidb-in-kubernetes/dev/access-tidb#gracefully-upgrade-the-tidb-cluster introduces the prestop hook for TiDB, however, we have to make sure that TiDB may be gracefully upgraded (the long connections will be down but no new connections will be rejected):

• For on-premise deploy, can the ingress controller meet the requirement? How to configure them?
• For public cloud deploy, can the LB + prestop hook meet the requirement?

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[DanielZhangQD]

Per the issue, https://github.com/kubernetes/kubernetes/issues/85643, for public cloud deploy, LB + Local externalTrafficPolicy cannot gracefully upgrade TiDB.