Warn when metrics are being overwritten

closes #601

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Features
 - [#564](https://github.com/influxdata/telegraf/issues/564): features for plugin writing simplification. Internal metric data type.
 - [#603](https://github.com/influxdata/telegraf/pull/603): Aggregate statsd timing measurements into fields. Thanks @marcinbunsch!
+- [#601](https://github.com/influxdata/telegraf/issues/601): Warn when overwriting cached metrics.
 
 ### Bugfixes
 - [#595](https://github.com/influxdata/telegraf/issues/595): graphite output should include tags to separate duplicate measurements.

internal/models/running_output.go

@@ -45,6 +45,9 @@ func (ro *RunningOutput) AddPoint(point telegraf.Metric) {
 	if len(ro.metrics) < ro.PointBufferLimit {
 		ro.metrics = append(ro.metrics, point)
 	} else {
+		log.Printf("WARNING: overwriting cached metrics, you may want to " +
+			"increase the metric_buffer_limit setting in your [agent] config " +
+			"if you do not wish to overwrite metrics.\n")
 		if ro.overwriteCounter == len(ro.metrics) {
 			ro.overwriteCounter = 0
 		}