Add BSD exec statistics collector (#457)

* First pass of a sysctl_bsd source, exec_bsd + exec metrics

* Incorportate PR feedback, including removing pre-build descriptions, unit conversion callback.

* Remove redundant cached_description field, per PR feedback

* Incorporate PR feedback

collector/exec_bsd.go

@@ -0,0 +1,98 @@
+// Copyright 2017 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 freebsd dragonfly
+// +build !noexec
+
+package collector
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type execCollector struct {
+	sysctls []bsdSysctl
+}
+
+func init() {
+	Factories["exec"] = NewExecCollector
+}
+
+// NewExecCollector returns a new Collector exposing system execution statistics
+func NewExecCollector() (Collector, error) {
+	// From sys/vm/vm_meter.c:
+	// All are of type CTLTYPE_UINT.
+	//
+	// vm.stats.sys.v_swtch: Context switches
+	// vm.stats.sys.v_trap: Traps
+	// vm.stats.sys.v_syscall: System calls
+	// vm.stats.sys.v_intr: Device interrupts
+	// vm.stats.sys.v_soft: Software interrupts
+	// vm.stats.vm.v_forks: Number of fork() calls
+
+	return &execCollector{
+		sysctls: []bsdSysctl{
+			{
+				name:        "context_switches_total",
+				description: "Context switches since system boot.  Resets at architeture unsigned integer.",
+				mib:         "vm.stats.sys.v_swtch",
+			},
+			{
+				name:        "traps_total",
+				description: "Traps since system boot.  Resets at architeture unsigned integer.",
+				mib:         "vm.stats.sys.v_trap",
+			},
+			{
+				name:        "system_calls_total",
+				description: "System calls since system boot.  Resets at architeture unsigned integer.",
+				mib:         "vm.stats.sys.v_syscall",
+			},
+			{
+				name:        "device_interrupts_total",
+				description: "Device interrupts since system boot.  Resets at architeture unsigned integer.",
+				mib:         "vm.stats.sys.v_intr",
+			},
+			{
+				name:        "software_interrupts_total",
+				description: "Software interrupts since system boot.  Resets at architeture unsigned integer.",
+				mib:         "vm.stats.sys.v_soft",
+			},
+			{
+				name:        "forks_total",
+				description: "Number of fork() calls since system boot.  Resets at architeture unsigned integer.",
+				mib:         "vm.stats.vm.v_forks",
+			},
+		},
+	}, nil
+}
+
+// Update pushes exec statistics onto ch
+func (c *execCollector) Update(ch chan<- prometheus.Metric) (err error) {
+	for _, m := range c.sysctls {
+		v, err := m.Value()
+		if err != nil {
+			return err
+		}
+
+		// We "know" all of our sysctls are CounterValues, let's skip
+		// parsing them
+		ch <- prometheus.MustNewConstMetric(
+			prometheus.NewDesc(
+				prometheus.BuildFQName(Namespace, "exec", m.name),
+				m.description,
+				nil, nil,
+			), prometheus.CounterValue, v)
+	}
+
+	return nil
+}