contrib/99designs/gqlgen: add WithCustomTag option

[samsullivan]

What does this PR do?

The gqlgen middleware was missing the ability to customize tags on the created spans. Implement a WithCustomTag option that follows the pattern of other similar packages.

I also renamed variable names for the config struct as cfg, instead of t, to match every other contrib package.

Motivation

Fixes #2597; supports the ability to use additional tagging with gqlgen traces.

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 Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[dianashevchenko]

@samsullivan Overall, looks good to me, but it would be nice to add a unit test 🙌

[samsullivan]

@samsullivan Overall, looks good to me, but it would be nice to add a unit test 🙌

I'll take a look at adding one shortly 👍

[samsullivan]

Pushed a new test case following the other option tests for this package, @dianashevchenko.