Notifications: Optional trace propagation through SMTP (#80481)

* Notifications: Optional trace propagation through SMTP

Signed-off-by: Dave Henderson <dave.henderson@grafana.com>

* fix failing test

Signed-off-by: Dave Henderson <dave.henderson@grafana.com>

* Add documentation

Signed-off-by: Dave Henderson <dave.henderson@grafana.com>

---------

Signed-off-by: Dave Henderson <dave.henderson@grafana.com>

pkg/setting/setting_smtp.go

@@ -20,6 +20,7 @@ type SmtpSettings struct {
 	StartTLSPolicy string
 	SkipVerify     bool
 	StaticHeaders  map[string]string
+	EnableTracing  bool
 
 	SendWelcomeEmailOnSignUp bool
 	TemplatesPatterns        []string
@@ -53,6 +54,8 @@ func (cfg *Cfg) readSmtpSettings() error {
 		return err
 	}
 
+	cfg.Smtp.EnableTracing = sec.Key("enable_tracing").MustBool(false)
+
 	return nil
 }