Add option for configuring the location of the container's host files…

…ystem

This allows the system module to read metrics related to the host machine rather than the container. When using the -system.hostfs flag, directories like /proc and /sys are assumed to be mounted relative to the path given in the value. Here's an example usage.

`docker run --volume=/:/hostfs:ro --net=host --name=metricbeat -d metricbeat -system.hostfs=/hostfs`

And here is a more restrictive example which does not mount all of the host's filesystems inside the container. system/filesystem will not be able to report metrics for the host's filesystem.

`docker run --volume=/proc:/hostfs/proc:ro --volume=/sys/fs/cgroup:/hostfs/sys/fs/cgroup --net=host --name=metricbeat -d metricbeat -system.hostfs=/hostfs`

metricbeat/module/system/process/process.go

@@ -3,12 +3,14 @@
 package process
 
 import (
+	"fmt"
 	"runtime"
 	"strconv"
 
 	"github.com/elastic/beats/libbeat/common"
 	"github.com/elastic/beats/libbeat/logp"
 	"github.com/elastic/beats/metricbeat/mb"
+	"github.com/elastic/beats/metricbeat/module/system"
 
 	"github.com/elastic/gosigar/cgroup"
 	"github.com/pkg/errors"
@@ -54,7 +56,12 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 	}
 
 	if runtime.GOOS == "linux" {
-		m.cgroup, err = cgroup.NewReader("", true)
+		systemModule, ok := base.Module().(*system.Module)
+		if !ok {
+			return nil, fmt.Errorf("unexpected module type")
+		}
+
+		m.cgroup, err = cgroup.NewReader(systemModule.HostFS, true)
 		if err != nil {
 			return nil, errors.Wrap(err, "error initializing cgroup reader")
 		}

metricbeat/module/system/system.go

@@ -1,6 +1,39 @@
 package system
 
-import "math"
+import (
+	"flag"
+	"math"
+	"sync"
+
+	"github.com/elastic/beats/metricbeat/mb"
+)
+
+var (
+	HostFS = flag.String("system.hostfs", "", "mountpoint of the host's filesystem for use in monitoring a host from within a container")
+)
+
+var once sync.Once
+
+func init() {
+	// Register the ModuleFactory function for the "system" module.
+	if err := mb.Registry.AddModule("system", NewModule); err != nil {
+		panic(err)
+	}
+}
+
+type Module struct {
+	mb.BaseModule
+	HostFS string // Mountpoint of the host's filesystem for use in monitoring inside a container.
+}
+
+func NewModule(base mb.BaseModule) (mb.Module, error) {
+	// This only needs to be configured once for all system modules.
+	once.Do(func() {
+		configureHostFS()
+	})
+
+	return &Module{BaseModule: base, HostFS: *HostFS}, nil
+}
 
 func Round(val float64, roundOn float64, places int) (newVal float64) {
 	var round float64

metricbeat/module/system/system_linux.go

@@ -0,0 +1,23 @@
+package system
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/elastic/gosigar"
+)
+
+func configureHostFS() {
+	dir := *HostFS
+	if dir == "" {
+		dir = "/"
+	}
+
+	// Set environment variables for gopsutil.
+	os.Setenv("HOST_PROC", filepath.Join(dir, "/proc"))
+	os.Setenv("HOST_SYS", filepath.Join(dir, "/sys"))
+	os.Setenv("HOST_ETC", filepath.Join(dir, "/etc"))
+
+	// Set proc location for gosigar.
+	gosigar.Procd = filepath.Join(dir, "/proc")
+}

metricbeat/module/system/system_other.go

@@ -0,0 +1,7 @@
+// +build !linux
+
+package system
+
+func configureHostFS() {
+	// Stub method for non-linux.
+}