Fix chrony plugin does not track system time offset (#2989)

plugins/inputs/chrony/README.md

@@ -63,6 +63,7 @@ Delete second or Not synchronised.
 ### Measurements & Fields:
 
 - chrony
+    - system_time (float, seconds)
     - last_offset (float, seconds)
     - rms_offset (float, seconds)
     - frequency (float, ppm)
@@ -84,7 +85,7 @@ Delete second or Not synchronised.
 ```
 $ telegraf --config telegraf.conf --input-filter chrony --test
 * Plugin: chrony, Collection 1
-> chrony,leap_status=normal,reference_id=192.168.1.1,stratum=3 frequency=-35.657,last_offset=-0.000013616,residual_freq=-0,rms_offset=0.000027073,root_delay=0.000644,root_dispersion=0.003444,skew=0.001,update_interval=1031.2 1463750789687639161
+> chrony,leap_status=normal,reference_id=192.168.1.1,stratum=3 frequency=-35.657,system_time=0.000027073,last_offset=-0.000013616,residual_freq=-0,rms_offset=0.000027073,root_delay=0.000644,root_dispersion=0.003444,skew=0.001,update_interval=1031.2 1463750789687639161
 ```
 
 

plugins/inputs/chrony/chrony.go

@@ -92,7 +92,7 @@ func processChronycOutput(out string) (map[string]interface{}, map[string]string
 		}
 		name := strings.ToLower(strings.Replace(strings.TrimSpace(stats[0]), " ", "_", -1))
 		// ignore reference time
-		if strings.Contains(name, "time") {
+		if strings.Contains(name, "ref_time") {
 			continue
 		}
 		valueFields := strings.Fields(stats[1])

plugins/inputs/chrony/chrony_test.go

@@ -31,6 +31,7 @@ func TestGather(t *testing.T) {
 		"stratum":      "3",
 	}
 	fields := map[string]interface{}{
+		"system_time":     0.000020390,
 		"last_offset":     0.000012651,
 		"rms_offset":      0.000025577,
 		"frequency":       -16.001,