Fix UnresolvedValueError in instance statistics

What does this MR do?

This change fixes the GraphQL::EnumType::UnresolvedValueError error when the identifier field is requested in the instance statistics graphql endpoint.

How to reproduce:

0. Make sure that we have seed data by running:
   • FILTER=instance_statistics bundle exec rake db:seed_fu
1. Log in as admin.
2. Go to graphql explorer
3. Execute the following query:

   {
     instanceStatisticsMeasurements(identifier: GROUPS, first: 1) {
       nodes {
         count
         recordedAt
         identifier
       }
     }
   }

Original error report: !40871 (comment 423596863)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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