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

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

expfmt/encode.go

@@ -116,7 +116,13 @@ 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 {
+//
+// NewEncoder can be called with additional options to customize the OpenMetrics text output.
+// For example:
+// NewEncoder(w, FmtOpenMetrics_1_0_0, WithCreatedLines())
+//
+// Extra options are ignored for all other formats.
+func NewEncoder(w io.Writer, format Format, options ...ToOpenMetricsOption) Encoder {
 	switch format {
 	case FmtProtoDelim:
 		return encoderCloser{
@@ -153,7 +159,7 @@ func NewEncoder(w io.Writer, format Format) Encoder {
 	case FmtOpenMetrics_0_0_1, FmtOpenMetrics_1_0_0:
 		return encoderCloser{
 			encode: func(v *dto.MetricFamily) error {
-				_, err := MetricFamilyToOpenMetrics(w, v)
+				_, err := MetricFamilyToOpenMetrics(w, v, options...)
 				return err
 			},
 			close: func() error {