Improve color accessibility of prometheus messaging

Problem:

In &896, we performed an audit of GitLab to ensure that we are following standard WCAG standards in terms of Use of Color.

1.4.1 Use of Color: Color is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element. (Level A)

In Monitor, when prometheus is being automatically managed, we have that information in green. Since this appears to be an instance where color is the only visual means of conveying information, we should probably add in an additional visual cue that all is functioning normally. Perhaps a green checkmark?