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_openbsd.go

@@ -0,0 +1,74 @@
+// 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
+
+package collector
+
+import (
+	"unsafe"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"golang.org/x/sys/unix"
+)
+
+/*
+#include <sys/types.h>
+#include <sys/disk.h>
+*/
+import "C"
+
+type diskstatsCollector struct {
+	rxfer  typedDesc
+	rbytes typedDesc
+	wxfer  typedDesc
+	wbytes typedDesc
+	time   typedDesc
+}
+
+func init() {
+	registerCollector("diskstats", defaultEnabled, NewDiskstatsCollector)
+}
+
+// NewDiskstatsCollector returns a new Collector exposing disk device stats.
+func NewDiskstatsCollector() (Collector, error) {
+	return &diskstatsCollector{
+		rxfer:  typedDesc{readsCompletedDesc, prometheus.CounterValue},
+		rbytes: typedDesc{readBytesDesc, prometheus.CounterValue},
+		wxfer:  typedDesc{writesCompletedDesc, prometheus.CounterValue},
+		wbytes: typedDesc{writtenBytesDesc, prometheus.CounterValue},
+		time:   typedDesc{ioTimeSecondsDesc, prometheus.CounterValue},
+	}, nil
+}
+
+func (c *diskstatsCollector) Update(ch chan<- prometheus.Metric) (err error) {
+	diskstatsb, err := unix.SysctlRaw("hw.diskstats")
+	if err != nil {
+		return err
+	}
+
+	ndisks := len(diskstatsb) / C.sizeof_struct_diskstats
+	diskstats := *(*[]C.struct_diskstats)(unsafe.Pointer(&diskstatsb))
+
+	for i := 0; i < ndisks; i++ {
+		diskname := C.GoString(&diskstats[i].ds_name[0])
+
+		ch <- c.rxfer.mustNewConstMetric(float64(diskstats[i].ds_rxfer), diskname)
+		ch <- c.rbytes.mustNewConstMetric(float64(diskstats[i].ds_rbytes), diskname)
+		ch <- c.wxfer.mustNewConstMetric(float64(diskstats[i].ds_wxfer), diskname)
+		ch <- c.wbytes.mustNewConstMetric(float64(diskstats[i].ds_wbytes), diskname)
+		time := float64(diskstats[i].ds_time.tv_sec) + float64(diskstats[i].ds_time.tv_usec)/1000000
+		ch <- c.time.mustNewConstMetric(time, diskname)
+	}
+	return nil
+}