Add durable_name to streaming subscription metrics

[Will2817]

[Will2817]

One question I wasn't sure about was: Since the durable_name is in the queue_name label when the subscription is a queue group and durable, should the queue_name label be split into durable_name and queue_name in this case?

[coveralls]

Coverage increased (+0.1%) to 90.247% when pulling 432bba2 on Will2817:export-subscription-durable-name into 873a29a on nats-io:master.

[wallyqs]

@Will2817 thanks for PR! splitting into queue_name and durable_name sounds like a good idea to me, would be useful to be able to query using different fields

[Will2817]

@wallyqs great, I've now updated the PR to also split the durable_name out from the queue_name when appropriate.

[wallyqs]

LGTM, thanks!