I have an apex class that creates a custom field on an sObject via an HttpRequest and I'm not sure how to write a test for this because one of the things I need to test is that the field has the correct value, so a mock or a bypass won't work. I can't get enough code coverage for this class if I exclude the web service call. Not sure where to go with this.

This is the request portion of the class in question:

```
public class RollupFieldCreator {

  @future(callout=true)
  public static void createRollupField(String fieldName, String description) {
      String fieldApiName = fieldName.trim().toLowerCase().replace(' ', '_') + '__c';

      if (!Schema.SObjectType.Asset.fields.getMap().keySet().contains(fieldApiName)) {
          String objectApiName = 'Asset';
          String toolingApiEndpoint = '/services/data/v41.0/tooling/sobjects/CustomField/';

          HttpRequest createField = new HttpRequest();
          createField.setHeader('Authorization', 'Bearer ' + UserInfo.getSessionId());
          createField.setHeader('Content-Type', 'application/json');
          createField.setEndpoint(URL.getSalesforceBaseUrl().toExternalForm() + toolingApiEndpoint);
          createField.setMethod('POST');
          String fieldDef = '{"Metadata" : {'
              + '"type" : "Text", "description" : "' + description
              + '", "inlineHelpText" : "", "precision" : null, "label" : "' + fieldName
              + '", "length" : 255, "required" : false}, '
              + '"FullName" : "' + objectApiName + '.' + fieldApiName + '"}';
          createField.setBody(fieldDef);
          Http httpInstance = new Http();
          HttpResponse response = httpInstance.send(createField);
          System.debug(response);
      }

```