Add metrics for messages sent to/received from clients

[alexbozhenko]

Add metrics that were added in nats-io/nats-server#7851

Since existing total counters(like nats_core_sent_bytes) already appear three times on the dashboard:

1. at the very top:

2. same metrics under the "cluster overview" section:

I thought that new metrics fit well under "network usage detail" pane, where we have grouped by the server

So I replaced total metrics with corresponding _clients metric: e.g. nats_core_sent_bytes with nats_core_sent_client_bytes