Unify CPU collector conventions (#806)

* Unify CPU collector conventions

Add a common CPU metric description.
* All collectors use the same `nodeCpuSecondsDesc`.
* All collectors drop the `cpu` prefix for `cpu` label values.

* Fix subsystem string in cpu_freebsd.

* Fix Linux CPU freq label names.

collector/cpu_common.go

@@ -0,0 +1,32 @@
+// Copyright 2016 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !nocpu
+
+package collector
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	cpuCollectorSubsystem = "cpu"
+)
+
+var (
+	nodeCpuSecondsDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, cpuCollectorSubsystem, "seconds_total"),
+		"Seconds the cpus spent in each mode.",
+		[]string{"cpu", "mode"}, nil,
+	)
+)