Labels modified by `pd-ctl` cannot be entirely persisted after restarting the cluster

## Enhancement Task
https://github.com/tikv/pd/pull/6468

1、If labels configured through `pd-ctl` conflict with those in the TiKV configuration file, they may be partially restored to match the labels in the TiKV configuration file after restarting the cluster. And the label after the restart is not entirely consistent with what is set through `pd-ctl` or the configuration in the TiKV configuration file.
Hope to optimize the merge strategy.

2、Modifying labels through `pd-ctl` does not persist entirely as described in the official documentation.
https://docs.pingcap.com/tidb/v6.5/pd-control#manage-store-labels
The doc might clarify the scenario in which the configuration in `pd-ctl` is fully merged and persisted only when the corresponding configuration in the TiKV configuration file is left blank.