expfmt/encoder: Allow opt-in for OM created lines

Signed-off-by: Arthur Silva Sens <arthur.sens@coralogix.com>

expfmt/encode.go

@@ -115,7 +115,12 @@ func NegotiateIncludingOpenMetrics(h http.Header) Format {
 // for FmtOpenMetrics, but a future (breaking) release will add the Close method
 // to the Encoder interface directly. The current version of the Encoder
 // interface is kept for backwards compatibility.
-func NewEncoder(w io.Writer, format Format) Encoder {
+func NewEncoder(w io.Writer, format Format, options ...ToOpenMetricsOption) Encoder {
+	toOM := toOpenMetrics{}
+	for _, opt := range options {
+		opt(&toOM)
+	}
+
 	switch format {
 	case FmtProtoDelim:
 		return encoderCloser{
@@ -150,6 +155,18 @@ func NewEncoder(w io.Writer, format Format) Encoder {
 			close: func() error { return nil },
 		}
 	case FmtOpenMetrics_0_0_1, FmtOpenMetrics_1_0_0:
+		if toOM.withCreatedLines {
+			return encoderCloser{
+				encode: func(v *dto.MetricFamily) error {
+					_, err := MetricFamilyToOpenMetrics(w, v, WithCreatedLines())
+					return err
+				},
+				close: func() error {
+					_, err := FinalizeOpenMetrics(w)
+					return err
+				},
+			}
+		}
 		return encoderCloser{
 			encode: func(v *dto.MetricFamily) error {
 				_, err := MetricFamilyToOpenMetrics(w, v)