Add "Number of Subjects" Metric

[tkeller-moxe]

What motivated this proposal?

We would like to track and create alerts on the number of subjects on a stream.
Something in our infrastructure has been having this not correctly correlate with the number of messages on a stream and it grows exponentially causing a memory leak. We would like to be able to monitor this for future regression and action.

What is the proposed change?

This Metric is available on the JSZ endpoint we would like this to be exported as a Prometheus metric.

Who benefits from this change?

NATs System Administrators and SMEs can leverage this datapoint to monitor system behavior from a single pane of glass instead of a CLI tool here or a webpage there.

What alternatives have you evaluated?

I can get this metric via the CLI or the web API myself. This is just the correct place to export it.

[Jarema]

We can definately add this metric.

Are you interested in contributing it?

[tkeller-moxe]

I was looking into contributing it but realized my knowledge of GO is substantially lacking. So I figured it made more sense to raise a request instead😅

[Jarema]

No worries :).

We will work on that soon.

[tkeller-moxe]

Thank you! :D