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

plugins/inputs/webhooks/github/github_webhooks_models.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
 )
 
 const meas = "github_webhooks"
@@ -106,7 +107,7 @@ func (s CommitCommentEvent) NewMetric() telegraf.Metric {
 		"commit":  s.Comment.Commit,
 		"comment": s.Comment.Body,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -136,7 +137,7 @@ func (s CreateEvent) NewMetric() telegraf.Metric {
 		"ref":     s.Ref,
 		"refType": s.RefType,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -166,7 +167,7 @@ func (s DeleteEvent) NewMetric() telegraf.Metric {
 		"ref":     s.Ref,
 		"refType": s.RefType,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -197,7 +198,7 @@ func (s DeploymentEvent) NewMetric() telegraf.Metric {
 		"environment": s.Deployment.Environment,
 		"description": s.Deployment.Description,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -231,7 +232,7 @@ func (s DeploymentStatusEvent) NewMetric() telegraf.Metric {
 		"depState":       s.DeploymentStatus.State,
 		"depDescription": s.DeploymentStatus.Description,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -259,7 +260,7 @@ func (s ForkEvent) NewMetric() telegraf.Metric {
 		"issues": s.Repository.Issues,
 		"fork":   s.Forkee.Repository,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -287,7 +288,7 @@ func (s GollumEvent) NewMetric() telegraf.Metric {
 		"forks":  s.Repository.Forks,
 		"issues": s.Repository.Issues,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -319,7 +320,7 @@ func (s IssueCommentEvent) NewMetric() telegraf.Metric {
 		"comments": s.Issue.Comments,
 		"body":     s.Comment.Body,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -351,7 +352,7 @@ func (s IssuesEvent) NewMetric() telegraf.Metric {
 		"title":    s.Issue.Title,
 		"comments": s.Issue.Comments,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -380,7 +381,7 @@ func (s MemberEvent) NewMetric() telegraf.Metric {
 		"newMember":       s.Member.User,
 		"newMemberStatus": s.Member.Admin,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -406,7 +407,7 @@ func (s MembershipEvent) NewMetric() telegraf.Metric {
 		"newMember":       s.Member.User,
 		"newMemberStatus": s.Member.Admin,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -432,7 +433,7 @@ func (s PageBuildEvent) NewMetric() telegraf.Metric {
 		"forks":  s.Repository.Forks,
 		"issues": s.Repository.Issues,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -458,7 +459,7 @@ func (s PublicEvent) NewMetric() telegraf.Metric {
 		"forks":  s.Repository.Forks,
 		"issues": s.Repository.Issues,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -495,7 +496,7 @@ func (s PullRequestEvent) NewMetric() telegraf.Metric {
 		"deletions":    s.PullRequest.Deletions,
 		"changedFiles": s.PullRequest.ChangedFiles,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -533,7 +534,7 @@ func (s PullRequestReviewCommentEvent) NewMetric() telegraf.Metric {
 		"commentFile":  s.Comment.File,
 		"comment":      s.Comment.Comment,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -565,7 +566,7 @@ func (s PushEvent) NewMetric() telegraf.Metric {
 		"before": s.Before,
 		"after":  s.After,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -593,7 +594,7 @@ func (s ReleaseEvent) NewMetric() telegraf.Metric {
 		"issues":  s.Repository.Issues,
 		"tagName": s.Release.TagName,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -619,7 +620,7 @@ func (s RepositoryEvent) NewMetric() telegraf.Metric {
 		"forks":  s.Repository.Forks,
 		"issues": s.Repository.Issues,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -649,7 +650,7 @@ func (s StatusEvent) NewMetric() telegraf.Metric {
 		"commit": s.Commit,
 		"state":  s.State,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -677,7 +678,7 @@ func (s TeamAddEvent) NewMetric() telegraf.Metric {
 		"issues":   s.Repository.Issues,
 		"teamName": s.Team.Name,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}
@@ -703,7 +704,7 @@ func (s WatchEvent) NewMetric() telegraf.Metric {
 		"forks":  s.Repository.Forks,
 		"issues": s.Repository.Issues,
 	}
-	m, err := telegraf.NewMetric(meas, t, f, time.Now())
+	m, err := metric.New(meas, t, f, time.Now())
 	if err != nil {
 		log.Fatalf("Failed to create %v event", event)
 	}