GraphQL client sends feature category header

• GraphQL calls return feature category and query file name to server
• Add labels from headers in web_transaction metrics
• Add x-gitlab-feature-category header to startup gql calls

What does this MR do?

EDIT:

This MR adds an ApolloLink which sets the x-gitlab-feature-category header to requests made from Apollo based on the gon.feature_category. This will be used to immediately help segment performance data for the GraphQL endpoint.

Originally, this MR also added x-gitlab-graphql-query-path, but this broke Apollo request batching ([see comment]*(!66937 (comment 652289139))). See the original description below for more details.

ORIGINAL:

This is a proof-of-concept to add feature category and query file name headers to add graphql calls.

Part of #336431 (closed)

Part of #328535

It adds two headers:

1. x-gitlab-feature-category
2. x-gitlab-graphql-query-path

Header names are up for discussion.

Next steps:

1. How do we configure CORS headers, change preflight responses for GitLab?
2. Add support for mutations

cc @grzesiek

Screenshots or Screencasts (strongly suggested)

EDIT:

When visiting a single issue on the local GDK:

Description	Screenshot
🎉 Header is added
🎉 Batching still works

ORIGINAL:

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)