expfmt/text: optionally include empty MetricFamilies

When parsing text format it is sometimes useful to keep all the
MetricFamilies around, even the ones that don't have any timeseries.
This is the case for the testutil library in client_golang where we need
to distinguish registered metrics from erronous inputs.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

expfmt/text_parse.go

@@ -78,6 +78,9 @@ type TextParser struct {
 	// These indicate if the metric name from the current line being parsed is inside
 	// braces and if that metric name was found respectively.
 	currentMetricIsInsideBraces, currentMetricInsideBracesIsPresent bool
+	// Include all MetricFamilies when parsing an input, even the ones for which
+	// the metric is registered but it doesn't have any timeseries.
+	includeEmptyMetricFamilies bool
 }
 
 // TextToMetricFamilies reads 'in' as the simple and flat text-based exchange
@@ -109,9 +112,11 @@ func (p *TextParser) TextToMetricFamilies(in io.Reader) (map[string]*dto.MetricF
 		// Magic happens here...
 	}
 	// Get rid of empty metric families.
-	for k, mf := range p.metricFamiliesByName {
-		if len(mf.GetMetric()) == 0 {
-			delete(p.metricFamiliesByName, k)
+	if !p.includeEmptyMetricFamilies {
+		for k, mf := range p.metricFamiliesByName {
+			if len(mf.GetMetric()) == 0 {
+				delete(p.metricFamiliesByName, k)
+			}
 		}
 	}
 	// If p.err is io.EOF now, we have run into a premature end of the input
@@ -124,6 +129,10 @@ func (p *TextParser) TextToMetricFamilies(in io.Reader) (map[string]*dto.MetricF
 	return p.metricFamiliesByName, p.err
 }
 
+func (p *TextParser) IncludeEmptyMetricFamilies(b bool) {
+	p.includeEmptyMetricFamilies = b
+}
+
 func (p *TextParser) reset(in io.Reader) {
 	p.metricFamiliesByName = map[string]*dto.MetricFamily{}
 	if p.buf == nil {