Add a way to check how busy the server is

In a common scenario, Bitbucket Server gets busy when CI's flood it with too frequent requests.
If Bitbucket Server could provide some way (e.g. REST API endpoint) to for the client-side to know how busy the server is at any given time, the CI's could be configured to check the server's status - and maybe wait a few minutes before initiating a heavy operation.

Right now, only Admins can monitor the server through JMX, but users and CI's have no way to check how busy the server is.

The workaround is for the Admin to publish the JMX metrics on a separate server, so users and CI's can find this information there. However, a light REST API endpoint (similar to the /status call) would be a better way to achieve this.