Use an internal 'Metric' data type for telegraf metrics

As of now, this is pretty much just a wrapper around client.Point, but
this gives latitude to expand functionality more easily.

closes #564

internal/models/metric.go

@@ -0,0 +1,87 @@
+package models
+
+import (
+	"time"
+
+	"github.com/influxdata/influxdb/client/v2"
+)
+
+type Metric interface {
+	// Name returns the measurement name of the metric
+	Name() string
+
+	// Name returns the tags associated with the metric
+	Tags() map[string]string
+
+	// Time return the timestamp for the metric
+	Time() time.Time
+
+	// UnixNano returns the unix nano time of the metric
+	UnixNano() int64
+
+	// 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
+}
+
+// metric is a wrapper of the influxdb client.Point struct
+type metric struct {
+	pt *client.Point
+}
+
+// NewMetric returns a metric with the given timestamp. If a timestamp is not
+// given, then data is sent to the database without a timestamp, in which case
+// the server will assign local time upon reception. NOTE: it is recommended to
+// send data with a timestamp.
+func NewMetric(
+	name string,
+	tags map[string]string,
+	fields map[string]interface{},
+	t ...time.Time,
+) (Metric, error) {
+	var T time.Time
+	if len(t) > 0 {
+		T = t[0]
+	}
+
+	pt, err := client.NewPoint(name, tags, fields, T)
+	if err != nil {
+		return nil, err
+	}
+	return &metric{
+		pt: pt,
+	}, nil
+}
+
+func (m *metric) Name() string {
+	return m.pt.Name()
+}
+
+func (m *metric) Tags() map[string]string {
+	return m.pt.Tags()
+}
+
+func (m *metric) Time() time.Time {
+	return m.pt.Time()
+}
+
+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)
+}