[ADDED] In/Out Msgs/Bytes in /serverz monitoring endpoint

[kozlovic]

Unlike total_msgs/bytes that represent the total number of messages
stored, in_msgs/bytes and out_msgs/bytes are the messages received
by the server and sent to clients.
The in_msgs counter is updated when processing inbound client
messages.
The out_msgs counter is updated when sending a message to a subscription,
either as a new or redelivered message.

Note that the size for in_bytes/out_bytes is different because these
values represent the bytes received (a PubMsg protobuf) and sent
(a MsgProto protobuf) that have different overhead.

Resolves #974

Signed-off-by: Ivan Kozlovic ivan@synadia.com

[kozlovic]

@ColinSullivan1 As requested from user in issue #974, I am adding in_msgs/in_bytes and out_msgs/out_bytes in streaming/serverz endpoint. This is similar to NATS core in/out msgs/bytes. Since the user is using the nats-exporter, I think that once this is merged, changes would have to be made in the nats-exporter to take advantage of those 4 new fields, right?

[renanberto]

It works beautifully. 👏

{
  "cluster_id": "local-nats",
  "server_id": "ptYgAQzCehetUfawkQqinl",
  "version": "0.16.2",
  "go": "go1.11.13",
  "state": "STANDALONE",
  "now": "2019-11-21T14:38:22.010618307Z",
  "start_time": "2019-11-21T14:35:08.744925427Z",
  "uptime": "3m13s",
  "clients": 1,
  "subscriptions": 47,
  "channels": 4255,
  "total_msgs": 500738,
  "total_bytes": 55940901,
  "in_msgs": 500738,
  "in_bytes": 64455885,
  "out_msgs": 0,
  "out_bytes": 0,
  "open_fds": 52,
  "max_fds": 1048576
}

@ColinSullivan1 As requested from user in issue #974, I am adding in_msgs/in_bytes and out_msgs/out_bytes in streaming/serverz endpoint. This is similar to NATS core in/out msgs/bytes. Since the user is using the nats-exporter, I think that once this is merged, changes would have to be made in the nats-exporter to take advantage of those 4 new fields, right?

@kozlovic @ColinSullivan1 Can you validate: nats-io/prometheus-nats-exporter#109 ?