prometheus state info collector uses state.ForModel

[Ian Booth]

The prometheus state info collector should use the state pool, not state.ForModel()

1 @ 0xd8cd0c 0xdd2204 0x1542e93 0x1544154 0x1543835 0x154358e 0xf19df1 0x45a7c1
#0xd8cd0bgithub.com/juju/juju/state.newState+0x2cb/home/nskaggs/gopath/src/github.com/juju/juju/state/open.go:593
#0xdd2203github.com/juju/juju/state.(*State).ForModel+0xa3/home/nskaggs/gopath/src/github.com/juju/juju/state/state.go:309
#0x1542e92github.com/juju/juju/state/statemetrics.stateShim.ForModel+0x42/home/nskaggs/gopath/src/github.com/juju/juju/state/statemetrics/state.go:103
#0x1544153github.com/juju/juju/state/statemetrics.(*Collector).updateModelMetrics+0x1c3/home/nskaggs/gopath/src/github.com/juju/juju/state/statemetrics/statemetrics.go:195
#0x1543834github.com/juju/juju/state/statemetrics.(*Collector).updateMetrics+0x1d4/home/nskaggs/gopath/src/github.com/juju/juju/state/statemetrics/statemetrics.go:146
#0x154358dgithub.com/juju/juju/state/statemetrics.(*Collector).Collect+0x1ad/home/nskaggs/gopath/src/github.com/juju/juju/state/statemetrics/statemetrics.go:128
#0xf19df0github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func2+0x60/home/nskaggs/gopath/src/github.com/prometheus/client_golang/prometheus/registry.go:382

[Andrew Wilkins (axwalk)]

https://github.com/juju/juju/pull/7509