Needs to know if it is ok to unconditionally drop classic histogram series

If we turn on NHCB, we need to parse classic values even if they are
dropped as classic series, to be able to do NHCB

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

model/textparse/interface.go

@@ -24,7 +24,9 @@ import (
 // Parser parses samples from a byte slice of samples in the official
 // Prometheus and OpenMetrics text exposition formats.
 type Parser interface {
-	Next(v *ExposedValues, d DropperCache) (ExposedMetricType, error)
+	// Parse returns the next metric with all information collected about it, such as
+	// metric type, help text, unit, created timestamps, samples, etc.
+	Next(v *ExposedValues, d DropperCache, keepClassicHistogramSeries bool) (ExposedMetricType, error)
 }
 
 type DropperCache interface {