Add number of users to 'system' plugin

see #235

CHANGELOG.md

@@ -15,6 +15,7 @@
 - [#799](https://github.com/influxdata/telegraf/pull/799): Add number of threads for procstat input plugin. Thanks @titilambert!
 - [#776](https://github.com/influxdata/telegraf/pull/776): Add Zookeeper chroot option to kafka_consumer. Thanks @prune998!
 - [#811](https://github.com/influxdata/telegraf/pull/811): Add processes plugin for classifying total procs on system. Thanks @titilambert!
+- [#235](https://github.com/influxdata/telegraf/issues/235): Add number of users to the `system` input plugin.
 
 ### Bugfixes
 - [#748](https://github.com/influxdata/telegraf/issues/748): Fix sensor plugin split on ":"

plugins/inputs/system/SYSTEM_README.md

+# System Input Plugin
+
+The system plugin gathers general stats on system load, uptime,
+and number of users logged in. It is basically equivalent
+to the unix `uptime` command.
+
+### Configuration:
+
+```toml
+# Read metrics about system load & uptime
+[[inputs.system]]
+  # no configuration
+```
+
+### Measurements & Fields:
+
+- system
+    - load1 (float)
+    - load15 (float)
+    - load5 (float)
+    - n_users (integer)
+    - uptime (integer, seconds)
+    - uptime_format (string)
+
+### Tags:
+
+None
+
+### Example Output:
+
+```
+$ telegraf -config ~/ws/telegraf.conf -input-filter system -test
+* Plugin: system, Collection 1
+> system load1=2.05,load15=2.38,load5=2.03,n_users=4i,uptime=239043i,uptime_format="2 days, 18:24" 1457546165399253452
+```

plugins/inputs/system/system.go

@@ -31,11 +31,17 @@ func (_ *SystemStats) Gather(acc telegraf.Accumulator) error {
 		return err
 	}
 
+	users, err := host.Users()
+	if err != nil {
+		return err
+	}
+
 	fields := map[string]interface{}{
 		"load1":         loadavg.Load1,
 		"load5":         loadavg.Load5,
 		"load15":        loadavg.Load15,
 		"uptime":        hostinfo.Uptime,
+		"n_users":       len(users),
 		"uptime_format": format_uptime(hostinfo.Uptime),
 	}
 	acc.AddFields("system", fields, nil)