Prometheus output: do not remake metrics map each write

closes #1775

CHANGELOG.md

@@ -23,13 +23,19 @@
 ### Bugfixes
 
 - [#1628](https://github.com/influxdata/telegraf/issues/1628): Fix mongodb input panic on version 2.2.
-- [#1738](https://github.com/influxdata/telegraf/issues/1738): Fix unmarshal of influxdb metrics with null tags
 - [#1733](https://github.com/influxdata/telegraf/issues/1733): Fix statsd scientific notation parsing
 - [#1716](https://github.com/influxdata/telegraf/issues/1716): Sensors plugin strconv.ParseFloat: parsing "": invalid syntax
 - [#1530](https://github.com/influxdata/telegraf/issues/1530): Fix prometheus_client reload panic
 - [#1764](https://github.com/influxdata/telegraf/issues/1764): Fix kafka consumer panic when nil error is returned down errs channel.
 - [#1768](https://github.com/influxdata/telegraf/pull/1768): Speed up statsd parsing.
 
+## v1.0.1 [unreleased]
+
+### Bugfixes
+
+- [#1775](https://github.com/influxdata/telegraf/issues/1775): Prometheus output: Fix bug with multi-batch writes.
+- [#1738](https://github.com/influxdata/telegraf/issues/1738): Fix unmarshal of influxdb metrics with null tags
+
 ## v1.0 [2016-09-08]
 
 ### Release Notes

plugins/outputs/prometheus_client/prometheus_client.go

@@ -17,7 +17,8 @@ var invalidNameCharRE = regexp.MustCompile(`[^a-zA-Z0-9_]`)
 type PrometheusClient struct {
 	Listen string
 
-	metrics map[string]prometheus.Metric
+	metrics     map[string]prometheus.Metric
+	lastMetrics map[string]prometheus.Metric
 
 	sync.Mutex
 }
@@ -28,6 +29,8 @@ var sampleConfig = `
 `
 
 func (p *PrometheusClient) Start() error {
+	p.metrics = make(map[string]prometheus.Metric)
+	p.lastMetrics = make(map[string]prometheus.Metric)
 	prometheus.Register(p)
 	defer func() {
 		if r := recover(); r != nil {
@@ -83,17 +86,24 @@ func (p *PrometheusClient) Collect(ch chan<- prometheus.Metric) {
 	p.Lock()
 	defer p.Unlock()
 
-	for _, m := range p.metrics {
-		ch <- m
+	if len(p.metrics) > 0 {
+		p.lastMetrics = make(map[string]prometheus.Metric)
+		for k, m := range p.metrics {
+			ch <- m
+			p.lastMetrics[k] = m
+		}
+		p.metrics = make(map[string]prometheus.Metric)
+	} else {
+		for _, m := range p.lastMetrics {
+			ch <- m
+		}
 	}
 }
 
 func (p *PrometheusClient) Write(metrics []telegraf.Metric) error {
 	p.Lock()
 	defer p.Unlock()
 
-	p.metrics = make(map[string]prometheus.Metric)
-
 	if len(metrics) == 0 {
 		return nil
 	}