fix tag name to be included

pkg/exporters/gelf_exporter/gelf.go

@@ -23,8 +23,6 @@ type GELFMessage struct {
 
 	// All additional field names must start with an underline.
 	ExtraFields map[string]interface{} `json:"extrafields,omitempty"`
-
-	Tag string `json:"tag,omitempty"`
 }
 
 func Log(writer *gelf.UDPWriter, msg GELFMessage) {
@@ -40,7 +38,6 @@ func Log(writer *gelf.UDPWriter, msg GELFMessage) {
 
 func (g GELFMessage) GELFFormat() *gelf.Message {
 	prefixedExtras := make(map[string]interface{}, len(g.ExtraFields))
-	prefixedExtras["tag"] = g.Tag
 	for k, v := range g.ExtraFields {
 		prefixedExtras["_"+k] = v
 	}