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/serializers/json/json_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/stretchr/testify/assert"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
 )
 
 func TestSerializeMetricFloat(t *testing.T) {
@@ -18,7 +19,7 @@ func TestSerializeMetricFloat(t *testing.T) {
 	fields := map[string]interface{}{
 		"usage_idle": float64(91.5),
 	}
-	m, err := telegraf.NewMetric("cpu", tags, fields, now)
+	m, err := metric.New("cpu", tags, fields, now)
 	assert.NoError(t, err)
 
 	s := JsonSerializer{}
@@ -36,7 +37,7 @@ func TestSerializeMetricInt(t *testing.T) {
 	fields := map[string]interface{}{
 		"usage_idle": int64(90),
 	}
-	m, err := telegraf.NewMetric("cpu", tags, fields, now)
+	m, err := metric.New("cpu", tags, fields, now)
 	assert.NoError(t, err)
 
 	s := JsonSerializer{}
@@ -55,7 +56,7 @@ func TestSerializeMetricString(t *testing.T) {
 	fields := map[string]interface{}{
 		"usage_idle": "foobar",
 	}
-	m, err := telegraf.NewMetric("cpu", tags, fields, now)
+	m, err := metric.New("cpu", tags, fields, now)
 	assert.NoError(t, err)
 
 	s := JsonSerializer{}
@@ -75,7 +76,7 @@ func TestSerializeMultiFields(t *testing.T) {
 		"usage_idle":  int64(90),
 		"usage_total": 8559615,
 	}
-	m, err := telegraf.NewMetric("cpu", tags, fields, now)
+	m, err := metric.New("cpu", tags, fields, now)
 	assert.NoError(t, err)
 
 	s := JsonSerializer{}