@Reference(dynamic = true, multiple = true, optional = true)
 public void addProvider(ObservationProvider<?> provider, Map<String, Object> properties) {
   String type = parseType(properties);
   if (type != null) {
     providers.put(provider, type);
     provider.subscribe(this);
   }
 }
 public void removeProvider(ObservationProvider<?> provider) {
   if (providers.remove(provider) != null) {
     provider.unsubscribe(this);
   }
 }