default value implementation

* default value config
* set default value in context source if metadata empty

fixes open-telemetry#34412

extension/headerssetterextension/config.go

@@ -19,10 +19,11 @@ type Config struct {
 }
 
 type HeaderConfig struct {
-	Action      ActionValue `mapstructure:"action"`
-	Key         *string     `mapstructure:"key"`
-	Value       *string     `mapstructure:"value"`
-	FromContext *string     `mapstructure:"from_context"`
+	Action       ActionValue `mapstructure:"action"`
+	Key          *string     `mapstructure:"key"`
+	Value        *string     `mapstructure:"value"`
+	FromContext  *string     `mapstructure:"from_context"`
+	DefaultValue *string     `mapstructure:"default_value"`
 }
 
 // ActionValue is the enum to capture the four types of actions to perform on a header

extension/headerssetterextension/extension.go

@@ -36,7 +36,8 @@ func newHeadersSetterExtension(cfg *Config, logger *zap.Logger) (auth.Client, er
 			}
 		} else if header.FromContext != nil {
 			s = &source.ContextSource{
-				Key: *header.FromContext,
+				Key:          *header.FromContext,
+				DefaultValue: *header.DefaultValue,
 			}
 		}
 

extension/headerssetterextension/internal/source/context.go

@@ -13,15 +13,16 @@ import (
 var _ Source = (*ContextSource)(nil)
 
 type ContextSource struct {
-	Key string
+	Key          string
+	DefaultValue string
 }
 
 func (ts *ContextSource) Get(ctx context.Context) (string, error) {
 	cl := client.FromContext(ctx)
 	ss := cl.Metadata.Get(ts.Key)
 
 	if len(ss) == 0 {
-		return "", nil
+		return ts.DefaultValue, nil
 	}
 
 	if len(ss) > 1 {