Rates as returned by GET /api/overview are always 0

[michaelklishin]

Originally reported as #265 without any details.

To reproduce, start a PerfTest instance and observe non-zero traffic flow and messages rates on the
connection(s), channel(s), and queue(s) pages but not Overview. Then try

curl -u guest:guest -X GET http://127.0.0.1:15672/api/overview | python -m json.tool | grep rate

The only relevant change in 3.6.4 seems to be #257, which resolved #250.

[michaelklishin]

Exchange rates are also not affected, so it's only GET /api/overview.

[Kelvin-Lei]

rabbitmqctl_environment.txt
server_log.txt

env: centos7, rabbitmq 3.6.4, erlang 19

I send a messge in 31-Jul-2016::19:40:03.

Queues tab is ok

but there is nothing to display in overview tab's charts