Monitor GraphQL with Prometheus

What does this MR do?

Enables tracing of GraphQL methods by creating Prometheus metrics.

It extends GraphQL's platform tracing class to observe duration of GraphQL methods.

Newer version of graphql-ruby includes prometheus class on top of platform tracing but it's not very useful for us (it uses prometheus-exporter), also our current code is not compatible with newer graphql-ruby version (yet).

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/48726

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer