Create public models for telegraf metrics, accumlator, plugins

This will basically make the root directory a place for storing the
major telegraf interfaces, which will make telegraf's godoc looks quite
a bit nicer. And make it easier for contributors to lookup the few data
types that they actually care about.

closes #564

plugins/outputs/prometheus_client/prometheus_client.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/influxdata/influxdb/client/v2"
 	"github.com/influxdata/telegraf/plugins/outputs"
+	"github.com/influxdata/telegraf"
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -119,7 +120,7 @@ func (p *PrometheusClient) Write(points []*client.Point) error {
 }
 
 func init() {
-	outputs.Add("prometheus_client", func() outputs.Output {
+	outputs.Add("prometheus_client", func() telegraf.Output {
 		return &PrometheusClient{}
 	})
 }