cgroup: improve documentation

This adds some documentation to the cgroup input plugin
to highlight potential influxdb cardinality issue with
a large number of cgroups.

CHANGELOG.md

@@ -54,6 +54,7 @@ continue sending logs to /var/log/telegraf/telegraf.log.
 - [#1833](https://github.com/influxdata/telegraf/issues/1833): Fix translating SNMP fields not in MIB.
 - [#1835](https://github.com/influxdata/telegraf/issues/1835): Fix SNMP emitting empty fields.
 - [#1854](https://github.com/influxdata/telegraf/pull/1853): SQL Server waitstats truncation bug.
+- [#1724](https://github.com/influxdata/telegraf/issues/1724): cgroups path being parsed as metric.
 
 ## v1.0.1 [2016-09-26]
 

plugins/inputs/cgroup/README.md

@@ -2,6 +2,10 @@
 
 This input plugin will capture specific statistics per cgroup.
 
+Consider restricting paths to the set of cgroups you really
+want to monitor if you have a large number of cgroups, to avoid
+any cardinality issues.
+
 Following file formats are supported:
 
 * Single value

plugins/inputs/cgroup/cgroup.go

@@ -12,6 +12,9 @@ type CGroup struct {
 
 var sampleConfig = `
     ## Directories in which to look for files, globs are supported.
+    ## Consider restricting paths to the set of cgroups you really
+    ## want to monitor if you have a large number of cgroups, to avoid
+    ## any cardinality issues.
 	# paths = [
 	#   "/cgroup/memory",
 	#   "/cgroup/memory/child1",