Add diskstats collector for OpenBSD (#1250)

* Add diskstats collector for OpenBSD

Tested on i386 and amd64, OpenBSD 6.4 and -current.

* Refactor diskstats collectors

This moves common descriptors from Linux, Darwin, OpenBSD
diskstats collectors into diskstats_common.go

Signed-off-by: Ralf Horstmann <ralf+github@ackstorm.de>

collector/diskstats_common.go

@@ -0,0 +1,73 @@
+// Copyright 2019 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 !nodiskstats
+// +build openbsd linux darwin
+
+package collector
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	diskSubsystem = "disk"
+)
+
+var (
+	diskLabelNames = []string{"device"}
+
+	readsCompletedDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, diskSubsystem, "reads_completed_total"),
+		"The total number of reads completed successfully.",
+		diskLabelNames, nil,
+	)
+
+	readBytesDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, diskSubsystem, "read_bytes_total"),
+		"The total number of bytes read successfully.",
+		diskLabelNames, nil,
+	)
+
+	writesCompletedDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, diskSubsystem, "writes_completed_total"),
+		"The total number of writes completed successfully.",
+		diskLabelNames, nil,
+	)
+
+	writtenBytesDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, diskSubsystem, "written_bytes_total"),
+		"The total number of bytes written successfully.",
+		diskLabelNames, nil,
+	)
+
+	ioTimeSecondsDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, diskSubsystem, "io_time_seconds_total"),
+		"Total seconds spent doing I/Os.",
+		diskLabelNames, nil,
+	)
+
+	readTimeSecondsDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, diskSubsystem, "read_time_seconds_total"),
+		"The total number of seconds spent by all reads.",
+		diskLabelNames,
+		nil,
+	)
+
+	writeTimeSecondsDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, diskSubsystem, "write_time_seconds_total"),
+		"This is the total number of seconds spent by all writes.",
+		diskLabelNames,
+		nil,
+	)
+)