fix: message formatting errors in GELF exporter

The library used to transport GELF messages did not prefix extra fields
with `_` as was previously assumed and the application name was not
passed correctly from configuration to the exporter. Now fixed.

Related: goc_marek/salego#693

pkg/exporters/gelf_exporter/gelf.go

@@ -31,10 +31,17 @@ func Log(writer *gelf.UDPWriter, msg GELFMessage) {
 		if err != nil {
 			log.Println(err)
 		}
+	} else {
+		log.Fatalln("gelf.UDPWriter is not set!")
 	}
 }
 
 func (g GELFMessage) GELFFormat() *gelf.Message {
+	prefixedExtras := make(map[string]interface{}, len(g.ExtraFields))
+	for k, v := range g.ExtraFields {
+		prefixedExtras["_"+k] = v
+	}
+
 	return &gelf.Message{
 		Version:  "1.1",
 		Host:     g.Host,
@@ -42,6 +49,6 @@ func (g GELFMessage) GELFFormat() *gelf.Message {
 		Full:     g.LongMessage,
 		TimeUnix: float64(g.Timestamp.Unix()),
 		Level:    int32(g.Level),
-		Extra:    g.ExtraFields,
+		Extra:    prefixedExtras,
 	}
 }

pkg/exporters/gelf_exporter/trace.go

@@ -29,6 +29,7 @@ func New(options ...Option) (*Exporter, error) {
 
 	return &Exporter{
 		gelfWriter: writer,
+		appName:    cfg.AppName,
 	}, nil
 
 }