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/running_aggregator.go

@@ -4,6 +4,7 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
 )
 
 type RunningAggregator struct {
@@ -90,7 +91,7 @@ func (r *RunningAggregator) Add(in telegraf.Metric) bool {
 			return false
 		}
 
-		in, _ = telegraf.NewMetric(name, tags, fields, t)
+		in, _ = metric.New(name, tags, fields, t)
 	}
 
 	r.metrics <- in