Add queue_name, is_durable, and is_offline labels to streaming subscription metrics

[andriy-bulynko]

Fixes #73

Subscription metrics labels would look like this:

# HELP nss_chan_subs_last_sent Last message sent
# TYPE nss_chan_subs_last_sent gauge
nss_chan_subs_last_sent{channel="channel",client_id="client-1",inbox="_INBOX.T2B88EAB82CMU1KA6CQNFN",is_durable="false",is_offline="false",queue_name="",server="http://localhost:8222"} 11
nss_chan_subs_last_sent{channel="channel",client_id="client-2",inbox="_INBOX.ETVIKB62MKRTCNS55BWHU0",is_durable="true",is_offline="true",queue_name="some-durable-sub-name:some-queue-name",server="http://localhost:8222"} 11
nss_chan_subs_last_sent{channel="channel",client_id="client-3",inbox="_INBOX.T6R1F9PP8XMY9B6X7WPN6T",is_durable="false",is_offline="false",queue_name="some-queue-name",server="http://localhost:8222"} 11
# HELP nss_chan_subs_max_inflight Max in flight message count
# TYPE nss_chan_subs_max_inflight gauge
nss_chan_subs_max_inflight{channel="channel",client_id="client-1",inbox="_INBOX.T2B88EAB82CMU1KA6CQNFN",is_durable="false",is_offline="false",queue_name="",server="http://localhost:8222"} 16384
nss_chan_subs_max_inflight{channel="channel",client_id="client-2",inbox="_INBOX.ETVIKB62MKRTCNS55BWHU0",is_durable="true",is_offline="true",queue_name="some-durable-sub-name:some-queue-name",server="http://localhost:8222"} 16384
nss_chan_subs_max_inflight{channel="channel",client_id="client-3",inbox="_INBOX.T6R1F9PP8XMY9B6X7WPN6T",is_durable="false",is_offline="false",queue_name="some-queue-name",server="http://localhost:8222"} 16384
# HELP nss_chan_subs_pending_count Pending message count
# TYPE nss_chan_subs_pending_count gauge
nss_chan_subs_pending_count{channel="channel",client_id="client-1",inbox="_INBOX.T2B88EAB82CMU1KA6CQNFN",is_durable="false",is_offline="false",queue_name="",server="http://localhost:8222"} 0
nss_chan_subs_pending_count{channel="channel",client_id="client-2",inbox="_INBOX.ETVIKB62MKRTCNS55BWHU0",is_durable="true",is_offline="true",queue_name="some-durable-sub-name:some-queue-name",server="http://localhost:8222"} 0
nss_chan_subs_pending_count{channel="channel",client_id="client-3",inbox="_INBOX.T6R1F9PP8XMY9B6X7WPN6T",is_durable="false",is_offline="false",queue_name="some-queue-name",server="http://localhost:8222"} 0

[wallyqs]

LGTM, thanks!