Exported JetStream disabled metric from /jsz

[mfaizanse]

Exported JetStream disabled metric from /jsz

If JetStream is disabled, then we get resp.disabled in the response of /jsz endpoint.

{
   "server_id":"NBKAYBIGPZ73E4LYEY64FU2BWTCGO53TTEYLJ5NVCP3DTM4H2DNQZFL6",
   "now":"2022-08-01T14:57:56.528474816Z",
   "disabled":true,
   "config":{
      "max_memory":0,
      "max_storage":0
   },
   "memory":0,
   "storage":0,
   "reserved_memory":0,
   "reserved_storage":0,
   "accounts":0,
   "ha_assets":0,
   "api":{
      "total":0,
      "errors":0
   },
   "streams":0,
   "consumers":0,
   "messages":0,
   "bytes":0
}

This PR exports that variable as nats_server_jetstream_disabled.

# HELP nats_server_jetstream_disabled JetStream disabled or not
# TYPE nats_server_jetstream_disabled gauge
nats_server_jetstream_disabled{cluster="xxx-nats",domain="",is_meta_leader="false",meta_leader="xxx-nats-1",server_id="NBUCPIJXXPLYEEKPF6C7EBZDFX3QCDHLEO2II4YMU5JEJXEAMOJ7AHJZ",server_name="xxx-nats-2"} 0

[wallyqs]

LGTM