Add a flag for the sysfs location.

Analogous to `collector.procfs`, but for `/sys`. Add tests for both.

collector/paths.go

@@ -0,0 +1,35 @@
+// Copyright 2015 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.
+
+package collector
+
+import (
+	"flag"
+	"path"
+
+	"github.com/prometheus/procfs"
+)
+
+var (
+	// The path of the proc filesystem.
+	procPath = flag.String("collector.procfs", procfs.DefaultMountPoint, "procfs mountpoint.")
+	sysPath  = flag.String("collector.sysfs", "/sys", "sysfs mountpoint.")
+)
+
+func procFilePath(name string) string {
+	return path.Join(*procPath, name)
+}
+
+func sysFilePath(name string) string {
+	return path.Join(*sysPath, name)
+}