Handle missing cgroups in Metricbeat (#3692)

Add error handling to system process metricset for when Linux cgroups are missing from the kernel.

Adds fix from elastic/gosigar#64.
Fixes #3666.

CHANGELOG.asciidoc

@@ -54,6 +54,7 @@ https://github.com/elastic/beats/compare/v5.1.1...master[Check the HEAD diff]
 - Fix go routine leak in docker module. {pull}3492[3492]
 - Fix bug docker module hanging when docker container killed. {issue}3610[3610]
 - Set timeout to period instead of 1s by default as documented.
+- Add error handling to system process metricset for when Linux cgroups are missing from the kernel. {pull}3692[3692]
 
 *Packetbeat*
 

glide.yaml

@@ -25,7 +25,7 @@ import:
   subpackages:
   - /difflib
 - package: github.com/elastic/gosigar
-  version: b49e01eb1e5c68c469392a63feaccae9352ceb12
+  version: v0.2.0
 - package: github.com/elastic/procfs
   version: abf152e5f3e97f2fafac028d2cc06c1feb87ffa5
 - package: github.com/samuel/go-parser

metricbeat/module/system/process/process.go

@@ -63,10 +63,14 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 		}
 
 		if config.Cgroups == nil || *config.Cgroups {
-			debugf("process cgroup data collection is enabled")
+			debugf("process cgroup data collection is enabled, using hostfs='%v'", systemModule.HostFS)
 			m.cgroup, err = cgroup.NewReader(systemModule.HostFS, true)
 			if err != nil {
-				return nil, errors.Wrap(err, "error initializing cgroup reader")
+				if err == cgroup.ErrCgroupsMissing {
+					logp.Warn("cgroup data collection will be disabled: %v", err)
+				} else {
+					return nil, errors.Wrap(err, "error initializing cgroup reader")
+				}
 			}
 		}
 	}