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

models/accumulator.go

@@ -0,0 +1,18 @@
+package models
+
+import "time"
+
+type Accumulator interface {
+	// Create a point with a value, decorating it with tags
+	// NOTE: tags is expected to be owned by the caller, don't mutate
+	// it after passing to Add.
+	Add(measurement string,
+		value interface{},
+		tags map[string]string,
+		t ...time.Time)
+
+	AddFields(measurement string,
+		fields map[string]interface{},
+		tags map[string]string,
+		t ...time.Time)
+}

models/input.go

@@ -0,0 +1,31 @@
+package models
+
+type Input interface {
+	// SampleConfig returns the default configuration of the Input
+	SampleConfig() string
+
+	// Description returns a one-sentence description on the Input
+	Description() string
+
+	// Gather takes in an accumulator and adds the metrics that the Input
+	// gathers. This is called every "interval"
+	Gather(Accumulator) error
+}
+
+type ServiceInput interface {
+	// SampleConfig returns the default configuration of the Input
+	SampleConfig() string
+
+	// Description returns a one-sentence description on the Input
+	Description() string
+
+	// Gather takes in an accumulator and adds the metrics that the Input
+	// gathers. This is called every "interval"
+	Gather(Accumulator) error
+
+	// Start starts the ServiceInput's service, whatever that may be
+	Start() error
+
+	// Stop stops the services and closes any necessary channels and connections
+	Stop()
+}

models/metric.go

@@ -0,0 +1,105 @@
+package models
+
+import (
+	"time"
+
+	"github.com/influxdata/influxdb/client/v2"
+	"github.com/influxdata/influxdb/models"
+)
+
+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
+}
+
+// ParseMetrics returns a slice of Metrics from a text representation of a
+// metric (in line-protocol format)
+// with each metric separated by newlines. If any metrics fail to parse,
+// a non-nil error will be returned in addition to the metrics that parsed
+// successfully.
+func ParseMetrics(buf []byte) ([]Metric, error) {
+	points, err := models.ParsePoints(buf)
+	metrics := make([]Metric, len(points))
+	for i, point := range points {
+		// Ignore error here because it's impossible that a model.Point
+		// wouldn't parse into client.Point properly
+		metrics[i], _ = NewMetric(point.Name(), point.Tags(),
+			point.Fields(), point.Time())
+	}
+	return metrics, err
+}
+
+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)
+}

models/output.go

@@ -0,0 +1,31 @@
+package models
+
+type Output interface {
+	// Connect to the Output
+	Connect() error
+	// Close any connections to the Output
+	Close() error
+	// Description returns a one-sentence description on the Output
+	Description() string
+	// SampleConfig returns the default configuration of the Output
+	SampleConfig() string
+	// Write takes in group of points to be written to the Output
+	Write(metrics []*Metric) error
+}
+
+type ServiceOutput interface {
+	// Connect to the Output
+	Connect() error
+	// Close any connections to the Output
+	Close() error
+	// Description returns a one-sentence description on the Output
+	Description() string
+	// SampleConfig returns the default configuration of the Output
+	SampleConfig() string
+	// Write takes in group of points to be written to the Output
+	Write(metrics []*Metric) error
+	// Start the "service" that will provide an Output
+	Start() error
+	// Stop the "service" that will provide an Output
+	Stop()
+}