Implement telegraf's own full metric type

main reasons behind this:
- make adding/removing tags cheap
- make adding/removing fields cheap
- make parsing cheaper
- make parse -> decorate -> write out bytes metric flow much faster

Refactor serializer to use byte buffer

internal/models/makemetric.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
 )
 
 // makemetric is used by both RunningAggregator & RunningInput
@@ -135,16 +136,7 @@ func makemetric(
 		}
 	}
 
-	var m telegraf.Metric
-	var err error
-	switch mType {
-	case telegraf.Counter:
-		m, err = telegraf.NewCounterMetric(measurement, tags, fields, t)
-	case telegraf.Gauge:
-		m, err = telegraf.NewGaugeMetric(measurement, tags, fields, t)
-	default:
-		m, err = telegraf.NewMetric(measurement, tags, fields, t)
-	}
+	m, err := metric.New(measurement, tags, fields, t, mType)
 	if err != nil {
 		log.Printf("Error adding point [%s]: %s\n", measurement, err.Error())
 		return nil