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

metric.go

@@ -3,8 +3,8 @@ package telegraf
 import (
 	"time"
 
+	// TODO remove
 	"github.com/influxdata/influxdb/client/v2"
-	"github.com/influxdata/influxdb/models"
 )
 
 // ValueType is an enumeration of metric types that represent a simple value.
@@ -19,178 +19,40 @@ const (
 )
 
 type Metric interface {
-	// Name returns the measurement name of the metric
+	Serialize() []byte
+	String() string // convenience function for string(Serialize())
+	Copy() Metric
+
+	// Tag functions
+	HasTag(key string) bool
+	AddTag(key, value string)
+	RemoveTag(key string) bool
+
+	// Field functions
+	HasField(key string) bool
+	AddField(key string, value interface{})
+	RemoveField(key string) bool
+
+	// Name functions
+	SetName(name string)
+	SetPrefix(prefix string)
+	SetSuffix(suffix string)
+
+	// Getting data structure functions
 	Name() string
-
-	// Name returns the tags associated with the metric
 	Tags() map[string]string
-
-	// Time return the timestamp for the metric
+	Fields() map[string]interface{}
 	Time() time.Time
-
-	// Type returns the metric type. Can be either telegraf.Gauge or telegraf.Counter
-	Type() ValueType
-
-	// UnixNano returns the unix nano time of the metric
 	UnixNano() int64
-
-	// HashID returns a non-cryptographic hash of the metric (name + tags)
-	// NOTE: do not persist & depend on this value to disk.
+	Type() ValueType
+	Len() int // returns the length of the serialized metric, including newline
 	HashID() uint64
 
-	// Fields returns the fields for the metric
-	Fields() map[string]interface{}
-
-	// String returns a line-protocol string of the metric
-	String() string
-
-	// PrecisionString returns a line-protocol string of the metric, at precision
-	PrecisionString(precison string) string
-
-	// Point returns a influxdb client.Point object
-	Point() *client.Point
-
-	// SetAggregate sets the metric's aggregate status
-	// This is so that aggregate metrics don't get re-sent to aggregator plugins
+	// aggregator things:
 	SetAggregate(bool)
-	// IsAggregate returns true if the metric is an aggregate
 	IsAggregate() bool
 
-	// Copy copies the metric
-	Copy() Metric
-}
-
-// metric is a wrapper of the influxdb client.Point struct
-type metric struct {
-	pt models.Point
-
-	mType ValueType
-
-	isaggregate bool
-}
-
-func NewMetricFromPoint(pt models.Point) Metric {
-	return &metric{
-		pt:    pt,
-		mType: Untyped,
-	}
-}
-
-// NewMetric returns an untyped metric.
-func NewMetric(
-	name string,
-	tags map[string]string,
-	fields map[string]interface{},
-	t time.Time,
-) (Metric, error) {
-	pt, err := models.NewPoint(name, models.NewTags(tags), fields, t)
-	if err != nil {
-		return nil, err
-	}
-	return &metric{
-		pt:    pt,
-		mType: Untyped,
-	}, nil
-}
-
-// NewGaugeMetric returns a gauge metric.
-// Gauge metrics should be used when the metric is can arbitrarily go up and
-// down. ie, temperature, memory usage, cpu usage, etc.
-func NewGaugeMetric(
-	name string,
-	tags map[string]string,
-	fields map[string]interface{},
-	t time.Time,
-) (Metric, error) {
-	pt, err := models.NewPoint(name, models.NewTags(tags), fields, t)
-	if err != nil {
-		return nil, err
-	}
-	return &metric{
-		pt:    pt,
-		mType: Gauge,
-	}, nil
-}
-
-// NewCounterMetric returns a Counter metric.
-// Counter metrics should be used when the metric being created is an
-// always-increasing counter. ie, net bytes received, requests served, errors, etc.
-func NewCounterMetric(
-	name string,
-	tags map[string]string,
-	fields map[string]interface{},
-	t time.Time,
-) (Metric, error) {
-	pt, err := models.NewPoint(name, models.NewTags(tags), fields, t)
-	if err != nil {
-		return nil, err
-	}
-	return &metric{
-		pt:    pt,
-		mType: Counter,
-	}, nil
-}
-
-func (m *metric) Name() string {
-	return m.pt.Name()
-}
-
-func (m *metric) Tags() map[string]string {
-	return m.pt.Tags().Map()
-}
-
-func (m *metric) Time() time.Time {
-	return m.pt.Time()
-}
-
-func (m *metric) Type() ValueType {
-	return m.mType
-}
-
-func (m *metric) HashID() uint64 {
-	return m.pt.HashID()
-}
-
-func (m *metric) UnixNano() int64 {
-	return m.pt.UnixNano()
-}
-
-func (m *metric) Fields() map[string]interface{} {
-	return m.pt.Fields()
-}
-
-func (m *metric) String() string {
-	return m.pt.String()
-}
-
-func (m *metric) PrecisionString(precison string) string {
-	return m.pt.PrecisionString(precison)
-}
-
-func (m *metric) Point() *client.Point {
-	return client.NewPointFrom(m.pt)
-}
-
-func (m *metric) IsAggregate() bool {
-	return m.isaggregate
-}
-
-func (m *metric) SetAggregate(b bool) {
-	m.isaggregate = b
-}
-
-func (m *metric) Copy() Metric {
-	t := time.Time(m.Time())
-
-	tags := make(map[string]string)
-	fields := make(map[string]interface{})
-	for k, v := range m.Tags() {
-		tags[k] = v
-	}
-	for k, v := range m.Fields() {
-		fields[k] = v
-	}
-
-	out, _ := NewMetric(m.Name(), tags, fields, t)
-	return out
+	// Point returns a influxdb client.Point object
+	// TODO remove this function
+	Point() *client.Point
 }