Add unit protobuf parser

Signed-off-by: Arianna Vespri <arianna.vespri@yahoo.it>

model/textparse/openmetricsparse.go

@@ -136,7 +136,6 @@ func (p *OpenMetricsParser) Type() ([]byte, MetricType) {
 // Must only be called after Next returned a unit entry.
 // The returned byte slices become invalid after the next call to Next.
 func (p *OpenMetricsParser) Unit() ([]byte, []byte) {
-	// The Prometheus format does not have units.
 	return p.l.b[p.offsets[0]:p.offsets[1]], p.text
 }
 

model/textparse/protobufparse.go

@@ -272,7 +272,7 @@ func (p *ProtobufParser) Type() ([]byte, MetricType) {
 // Unit always returns (nil, nil) because units aren't supported by the protobuf
 // format.
 func (p *ProtobufParser) Unit() ([]byte, []byte) {
-	return nil, nil
+	return p.metricBytes.Bytes(), []byte(p.mf.GetUnit())
 }
 
 // Comment always returns nil because comments aren't supported by the protobuf