code: public Observable<TagDetailsInner> createOrUpdateAsync(String tagName) {
          return createOrUpdateWithServiceResponseAsync(tagName).map(new Func1<ServiceResponse<TagDetailsInner>, TagDetailsInner>() {
              @Override
              public TagDetailsInner call(ServiceResponse<TagDetailsInner> response) {
                  return response.body();
              }
          });
      }
docs: Creates a tag in the subscription.
  The tag name can have a maximum of 512 characters and is case insensitive. Tag names created by Azure have prefixes of microsoft, azure, or windows. You cannot create tags with one of these prefixes.
  
  @param tagName The name of the tag to create.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the TagDetailsInner object