Update prometheus-grafana.md and add grafana support allowed_origins

[bertreyking]

Why are these changes needed?

• when i accessing raydashboard, the browser encounters a cross-origin access issue when attempting to access Grafana.
• grafana.ini add the '''allowed_origins = *''' sloved

[anyscalesam]

@bertreyking does this block grafana from loading or is this just an error/warning output in dev tools?

[bertreyking]

@anyscalesam does this block grafana from loading or is this just an error/warning output in dev tools?

• It will not prevent the loading of grafana configuration. For details, please refer to grafana_configure
• Currently only the dev environment is used for verification.

[anyscalesam]

boop @alanwguo @kevin85421 all Qs look to be addressed; can you approve here so we can merge?