If the user has disabled Prometheus metrics and a request is
sent to the metrics endpoint requesting Prometheus formatted
metrics, then the request should fail.
The test asserts that alloc counts get reported accurately in metrics by
inspecting the metrics endpoint directly. Sadly, the metrics as
collected by `armon/go-metrics` seem to be stateful and may contain info
from other tests.
This means that the test can fail depending on the order of returned
metrics.
Inspecting the metrics output of one failing run, you can see the
duplicate guage entries but for different node_ids:
```
{
"Name": "service-name.default-0a3ba4b6-2109-485e-be74-6864228aed3d.client.allocations.terminal",
"Value": 10,
"Labels": {
"datacenter": "dc1",
"node_class": "none",
"node_id": "67402bf4-00f3-bd8d-9fa8-f4d1924a892a"
}
},
{
"Name": "service-name.default-0a3ba4b6-2109-485e-be74-6864228aed3d.client.allocations.terminal",
"Value": 0,
"Labels": {
"datacenter": "dc1",
"node_class": "none",
"node_id": "a2945b48-7e66-68e2-c922-49b20dd4e20c"
}
},
```
* client/metrics: modified metrics to use (updated) client copy of allocation instead of (unupdated) server copy
* updated armon/go-metrics to address race condition in DisplayMetrics
