Implement a per-output fixed size metric buffer

Also moved some objects out of config.go and put them in their own
package, internal/models

fixes #568
closes #285

internal/models/running_output.go

@@ -0,0 +1,77 @@
+package models
+
+import (
+	"log"
+	"time"
+
+	"github.com/influxdata/telegraf/plugins/outputs"
+
+	"github.com/influxdata/influxdb/client/v2"
+)
+
+const DEFAULT_POINT_BUFFER_LIMIT = 10000
+
+type RunningOutput struct {
+	Name             string
+	Output           outputs.Output
+	Config           *OutputConfig
+	Quiet            bool
+	PointBufferLimit int
+
+	points           []*client.Point
+	overwriteCounter int
+}
+
+func NewRunningOutput(
+	name string,
+	output outputs.Output,
+	conf *OutputConfig,
+) *RunningOutput {
+	ro := &RunningOutput{
+		Name:             name,
+		points:           make([]*client.Point, 0),
+		Output:           output,
+		Config:           conf,
+		PointBufferLimit: DEFAULT_POINT_BUFFER_LIMIT,
+	}
+	return ro
+}
+
+func (ro *RunningOutput) AddPoint(point *client.Point) {
+	if ro.Config.Filter.IsActive {
+		if !ro.Config.Filter.ShouldPointPass(point) {
+			return
+		}
+	}
+
+	if len(ro.points) < ro.PointBufferLimit {
+		ro.points = append(ro.points, point)
+	} else {
+		if ro.overwriteCounter == len(ro.points) {
+			ro.overwriteCounter = 0
+		}
+		ro.points[ro.overwriteCounter] = point
+		ro.overwriteCounter++
+	}
+}
+
+func (ro *RunningOutput) Write() error {
+	start := time.Now()
+	err := ro.Output.Write(ro.points)
+	elapsed := time.Since(start)
+	if err == nil {
+		if !ro.Quiet {
+			log.Printf("Wrote %d metrics to output %s in %s\n",
+				len(ro.points), ro.Name, elapsed)
+		}
+		ro.points = make([]*client.Point, 0)
+		ro.overwriteCounter = 0
+	}
+	return err
+}
+
+// OutputConfig containing name and filter
+type OutputConfig struct {
+	Name   string
+	Filter Filter
+}