Support nats-streaming server metrics

[mitchhh22]

I'm looking for a solution to monitoring the /streaming metrics exposed by a nats-streaming server.

For example: http://localhost:8222/streaming/clientsz?limit=1&offset=1&subs=1

Specifically it would be great to have prometheus scrape the pending_count metric for each subscription. Trying to monitor this currently yields a skewed result due to the many messages in/out between nodes in the nats-streaming cluster that aren't from connected clients.

[ColinSullivan1]

@mitchhh22 , This sounds good, and I agree that we should include streaming metrics in the prometheus monitor. We'll look into it.

[wallyqs]

This was supported via #54