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/sysctl_bsd.go

@@ -0,0 +1,70 @@
+// 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 !nomeminfo
+
+package collector
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"golang.org/x/sys/unix"
+)
+
+type bsdSysctlType uint8
+
+// BSD-specific sysctl value types.  There is an impedience mismatch between
+// native C types, e.g. int vs long, and the golang unix.Sysctl variables
+
+const (
+	// Default to uint32.
+	bsdSysctlTypeUint32 bsdSysctlType = iota
+	bsdSysctlTypeUint64
+)
+
+// Contains all the info needed to map a single bsd-sysctl to a prometheus value.
+type bsdSysctl struct {
+	// Prometheus name
+	name string
+
+	// Simple prometheus description
+	description string
+
+	// Prometheus type
+	valueType prometheus.ValueType
+
+	// Sysctl name
+	mib string
+
+	// Sysctl data-type
+	dataType bsdSysctlType
+}
+
+func (b bsdSysctl) Value() (float64, error) {
+	var tmp32 uint32
+	var tmp64 uint64
+	var err error
+
+	switch b.dataType {
+	case bsdSysctlTypeUint32:
+		tmp32, err = unix.SysctlUint32(b.mib)
+		tmp64 = uint64(tmp32)
+	case bsdSysctlTypeUint64:
+		tmp64, err = unix.SysctlUint64(b.mib)
+	}
+	if err != nil {
+		return 0, err
+	}
+
+	return float64(tmp64), nil
+}