Suggestion to add additional metrics for nats server

[YashasAnand]

As per my discussion with @mtmk in https://github.com/nats-io/nats.net/discussions I am suggesting to add these below metrics if possible & would like to know if this is feasible.

1. In Jetstream, if i have 1 subject & 10 consumers bound to the same subject, Nats server considers this as 1 consumer, but internally does load-balancing amongst all consumer threads. Could we have a metric like NumClientsForConsumer which can give us how many clients are attached to a consumers which can give good visibility in microservice based setups , especially at times when load is high & we want to scale consumers.

2. In a set-up where jetstream & core-nats are used together, I would like to see some more metrics on the core-nats subscribers like last consumed message on subscriber. Again, in a microservice architecture, it will be difficult to maintain our own caches to get the subscribers last consumed message, if this can be a metric available on server, it can give great visisbility.