Consider enable prometheusReloader and reloader by default

[just1900]

trafficstars

Feature Request

The feature of reloading prometheus rules and config files are commonly needed, can we enable it by default rather than configure it in tidbmonitor.spec?

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

Describe the feature you'd like: enable PrometheusReloader and reloader by default.

Describe alternatives you've considered:

• merge prometheusReloader and reloader's function into one container to reload rules and configs.(this is what prometheus-config-reloader is doing now) But we should consider that reloader also provide an ui for rules configuration

Teachability, Documentation, Adoption, Migration Strategy:

[mikechengwei]

Operator upgrade will cause tidbmonitor to restart. I think this effect is okay.