contrib/99designs/gqlgen: fix introspection query detection by checking Operation.Name

[LamineKouissi]

What does this PR do?

contrib/99designs/gqlgen: fix introspection query detection by checking Operation.Name

Motivation

InterceptField() relied on OperationName to identify introspection queries. This was insufficient as valid introspection queries can have an unset OperationName while Operation.Name is set to IntrospectionQuery.

Could Fix #3085

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

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