Metrics refactor

[banks]

See #68

Not sure about name counters but it's as good as any I came up with.

I tested this ad-hoc by opening three terminals and these commands, on in each:

$ ./centrifugo --insecure_api
$ watch -n 1 -d 'curl -s -X POST -H "Content-Type: application/json" -d "[{\"method\":\"counters\",\"params\":{}}]" localhost:8000/api/ | jq "."'
$ watch -n 1 -d 'curl -s -X POST -H "Content-Type: application/json" -d "[{\"method\":\"stats\",\"params\":{}}]" localhost:8000/api/ | jq "."'

See

You can see the api requests counter has changed in the 1 second since last update in the middle (counters command) and not changed on the right (stats command). Counters command also shows cumulative total, while stats only last minute (which is ~120 since there are two watchers running about every 1 second).

[banks]

@FZambia now it's node :)

Also note: I removed go-metrics from Godep file by hand because I'm actually running go1.6rc and didn't want to save that in Godep file.

I noticed though that there are a couple of other packages in Godep file that godep diff seems to think are no longer used in the code - in a separate PR/branch would be good to clean those up at some point.

[FZambia]

@banks wonderful! Thanks a lot!