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/outputs/instrumental/instrumental_test.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -26,13 +27,13 @@ func TestWrite(t *testing.T) {
 	}
 
 	// Default to gauge
-	m1, _ := telegraf.NewMetric(
+	m1, _ := metric.New(
 		"mymeasurement",
 		map[string]string{"host": "192.168.0.1"},
 		map[string]interface{}{"myfield": float64(3.14)},
 		time.Date(2010, time.November, 10, 23, 0, 0, 0, time.UTC),
 	)
-	m2, _ := telegraf.NewMetric(
+	m2, _ := metric.New(
 		"mymeasurement",
 		map[string]string{"host": "192.168.0.1", "metric_type": "set"},
 		map[string]interface{}{"value": float64(3.14)},
@@ -43,27 +44,27 @@ func TestWrite(t *testing.T) {
 	i.Write(metrics)
 
 	// Counter and Histogram are increments
-	m3, _ := telegraf.NewMetric(
+	m3, _ := metric.New(
 		"my_histogram",
 		map[string]string{"host": "192.168.0.1", "metric_type": "histogram"},
 		map[string]interface{}{"value": float64(3.14)},
 		time.Date(2010, time.November, 10, 23, 0, 0, 0, time.UTC),
 	)
 	// We will modify metric names that won't be accepted by Instrumental
-	m4, _ := telegraf.NewMetric(
+	m4, _ := metric.New(
 		"bad_metric_name",
 		map[string]string{"host": "192.168.0.1:8888::123", "metric_type": "counter"},
 		map[string]interface{}{"value": 1},
 		time.Date(2010, time.November, 10, 23, 0, 0, 0, time.UTC),
 	)
 	// We will drop metric values that won't be accepted by Instrumental
-	m5, _ := telegraf.NewMetric(
+	m5, _ := metric.New(
 		"bad_values",
 		map[string]string{"host": "192.168.0.1", "metric_type": "counter"},
 		map[string]interface{}{"value": "\" 3:30\""},
 		time.Date(2010, time.November, 10, 23, 0, 0, 0, time.UTC),
 	)
-	m6, _ := telegraf.NewMetric(
+	m6, _ := metric.New(
 		"my_counter",
 		map[string]string{"host": "192.168.0.1", "metric_type": "counter"},
 		map[string]interface{}{"value": float64(3.14)},