Support exposing current metric value on server check command

[samuel-form3]

Current behaviour

Currently when we execute the nats server check command, the client expects a set of threshold flags as inputs to be able to answer if the server is healthy or not according to the provided thresholds.

Feature request

It would be useful to know get current metric values, instead of just knowing if the threshold was exceeded or not.

This would enable us to create a prometheus exporter component that could export prometheus metrics on all of the nats server check set of commands. We would use these metrics on alerts and define the needed thresholds on the alerts themselves.

Desired behaviour

• Optional threshold flags, because those thresholds could be set on the alerts.
• New metrics on prometheus format to show the current health state.

Example

Currently

nats server check stream --server nats://nats:4222 \
  --stream TEST \
  --peer-expect 1 \ 
  --lag-critical 100 \
  --msgs-warn 4000 \
  --msgs-critical 3000 \
  --min-sources 33 \
  --max-sources 34 \
  --peer-lag-critical 100 \
  --peer-seen-critical 5m \
  --format prometheus

Example output:

# HELP nats_server_check_stream_peer_lagged RAFT peers that are lagged more than configured threshold
# TYPE nats_server_check_stream_peer_lagged gauge
nats_server_check_stream_peer_lagged{item="TEST"} 0
...

Proposed

nats server check stream --server nats://nats:4222 \
  --stream TEST \
  --format prometheus

In the previous example it would export a metric saying if a given peer has lag according to the provided threshold flag --peer-lag-critical 100. In this example, it would just export the peer lag itself for each peer <> stream.

This strategy could be applied for every other type of metric currently available on the tool.

Example output:

# HELP nats_server_check_stream_peer_lag RAFT peer lag
# TYPE nats_server_check_stream_peer_lag gauge
nats_server_check_stream_peer_lag{item="TEST", peer="nats-2"} 200
...

Thanks :pray: