Provide more detail about service state during startup/jruby instance creation

[Alives]

/status/v1/services seems to return running state once the first jruby instance is started. I'd prefer to wait until all of my instances have started before exposing that the service is healthy. 1 instance cannot handle the flood of traffic soon to come its way.

/status/v1/services?level=debug gives more detail including:

• num-free-jrubies, but if there is already traffic then this count will be lower than the available jrubies.
• num-jrubies seems to be the total configured to run, equivalent to max-active-instances in puppetserver.conf

Maybe if another metric could be exposed here of num-initialized-jrubies or equivalent, healthcheck could monitor that. And maybe also a way for the basic /status/v1/services to return startup instead of running until num-initialized-jrubies == num-jrubies while in a startup mode.

Are there currently any ways to prevent traffic until the service is ready regarding these contexts?

[Alives]

This is a dupe of #2844