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

plugins/inputs/logparser/grok/grok.go

@@ -13,6 +13,7 @@ import (
 	"github.com/vjeantet/grok"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
 )
 
 var timeLayouts = map[string]string{
@@ -280,7 +281,7 @@ func (p *Parser) ParseLine(line string) (telegraf.Metric, error) {
 		}
 	}
 
-	return telegraf.NewMetric(p.Measurement, tags, fields, p.tsModder.tsMod(timestamp))
+	return metric.New(p.Measurement, tags, fields, p.tsModder.tsMod(timestamp))
 }
 
 func (p *Parser) addCustomPatterns(scanner *bufio.Scanner) {