receiver/prometheus: add ToMetricPdata method

This change wires together the logic to convert
the pdata converters into a method ToMetricPdata
that appends converted metrics. The next change or two,
will then hook everything up directly and will
allow us to delete all the prior code!!

Updates #3137
Updates PR open-telemetry#3674
Requires PR open-telemetry#3694
Updates open-telemetry#3691

receiver/prometheusreceiver/internal/otlp_metricsbuilder.go

@@ -24,9 +24,9 @@ import (
 	"github.com/prometheus/prometheus/pkg/labels"
 	"github.com/prometheus/prometheus/pkg/textparse"
 	"github.com/prometheus/prometheus/pkg/value"
-	"go.uber.org/zap"
 
 	"go.opentelemetry.io/collector/model/pdata"
+	"go.uber.org/zap"
 )
 
 func isUsefulLabelPdata(mType pdata.MetricDataType, labelKey string) bool {
@@ -110,10 +110,6 @@ func newMetricBuilderPdata(mc MetadataCache, useStartTimeMetric bool, startTimeM
 	}
 }
 
-// This code is used in follow-up changes but golangci-lint is so pedantic.
-var _ = newMetricBuilderPdata
-var _ = (*metricBuilderPdata)(nil).AddDataPoint
-
 // AddDataPoint is for feeding prometheus data complexValue in its processing order
 func (b *metricBuilderPdata) AddDataPoint(ls labels.Labels, t int64, v float64) (rerr error) {
 	// Any datapoint with duplicate labels MUST be rejected per: