Replace deprecated Counter.Set with Gauge.Set

[sekikn]

Tried to build hadoop_exporter against the master of prometheus/client_golang, but it failed with the following error:

$ go build namenode_exporter.go
# command-line-arguments
./namenode_exporter.go:243:15: e.pnGcCount.Set undefined (type prometheus.Counter has no field or method Set)
./namenode_exporter.go:244:14: e.pnGcTime.Set undefined (type prometheus.Counter has no field or method Set)
./namenode_exporter.go:247:16: e.cmsGcCount.Set undefined (type prometheus.Counter has no field or method Set)
./namenode_exporter.go:248:15: e.cmsGcTime.Set undefined (type prometheus.Counter has no field or method Set)

$ go build resourcemanager_exporter.go 
# command-line-arguments
./resourcemanager_exporter.go:256:17: e.appsCompleted.Set undefined (type prometheus.Counter has no field or method Set)
./resourcemanager_exporter.go:257:17: e.appsSubmitted.Set undefined (type prometheus.Counter has no field or method Set)

This is because Set method for Counter type has been removed on the master.

https://github.com/prometheus/client_golang/pull/247/commits/2fee50beaa152342e8347c41edf2202c3bd5f858

I think there's no problem if we use Gauge for these metrics instead of Counter.

[wyukawa]

Thanks!