Support scheduler visibility and management

[kolbe]

Feature Request

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

It's often difficult to diagnose issues with schedulers and maintain their activation and configuration. This requires the use of pd-ctl, which is not very convenient especially when running in Kubernetes.

Describe the feature you'd like:

I'd like it to be possible to see which schedulers (https://docs.pingcap.com/tidb/stable/glossary#scheduler, https://docs.pingcap.com/tidb/stable/pd-control#scheduler-show--add--remove--pause--resume--config) are enabled in the Dashboard and to turn them on/off and change their configurations using a simple web UI.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[kennytm]

As of tikv/pd#3055 we can list enabled and paused schedulers. However there is no API to list all disabled schedulers, which means we gotta hard-code the built-in scheduler names.

• balance-leader-scheduler
• balance-hot-region-scheduler
• balance-region-scheduler
• label-scheduler
• scatter-range
• balance-adjacent-region-scheduler
• grant-leader-scheduler
• evict-leader-scheduler
• shuffle-leader-scheduler
• shuffle-region-scheduler
• random-merge-scheduler
• shuffle-hot-region-scheduler

[kolbe]

Can't we simply add an API to list all available schedulers?

[breezewish]

@HunDunDM Can we have an API for all schedulers?

[HunDunDM]

Some schedulers have been deprecated, and some are for testing. I think we need to discuss.