Add prometheus serializer and use it in prometheus output (#6703)

plugins/serializers/prometheus/prometheus.go

@@ -0,0 +1,69 @@
+package prometheus
+
+import (
+	"bytes"
+
+	"github.com/influxdata/telegraf"
+	"github.com/prometheus/common/expfmt"
+)
+
+// TimestampExport controls if the output contains timestamps.
+type TimestampExport int
+
+const (
+	NoExportTimestamp TimestampExport = iota
+	ExportTimestamp
+)
+
+// MetricSortOrder controls if the output is sorted.
+type MetricSortOrder int
+
+const (
+	NoSortMetrics MetricSortOrder = iota
+	SortMetrics
+)
+
+// StringHandling defines how to process string fields.
+type StringHandling int
+
+const (
+	DiscardStrings StringHandling = iota
+	StringAsLabel
+)
+
+type FormatConfig struct {
+	TimestampExport TimestampExport
+	MetricSortOrder MetricSortOrder
+	StringHandling  StringHandling
+}
+
+type Serializer struct {
+	config FormatConfig
+}
+
+func NewSerializer(config FormatConfig) (*Serializer, error) {
+	s := &Serializer{config: config}
+	return s, nil
+}
+
+func (s *Serializer) Serialize(metric telegraf.Metric) ([]byte, error) {
+	return s.SerializeBatch([]telegraf.Metric{metric})
+}
+
+func (s *Serializer) SerializeBatch(metrics []telegraf.Metric) ([]byte, error) {
+	coll := NewCollection(s.config)
+	for _, metric := range metrics {
+		coll.Add(metric)
+	}
+
+	var buf bytes.Buffer
+	for _, mf := range coll.GetProto() {
+		enc := expfmt.NewEncoder(&buf, expfmt.FmtText)
+		err := enc.Encode(mf)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return buf.Bytes(), nil
+}