Add client-only message/byte metrics and use them in Network Usage Detail

[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:

3. under "network usage detail"

I think we can just replace metric "network usage detail" pane, where we have data grouped by the server

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

[Jarema]

@alexbozhenko please resolve the conflicts.