Apply dedicated rate limit to create snippet APIs

What does this MR do and why?

Apply dedicated rate limit to create snippet APIs

• POST api/v4/snippets
• POST api/v4/projects/:id/snippets
• CreateSnippet GraphQL mutation

Changelog: added

Usage analysis: https://gitlab.com/gitlab-org/gitlab/-/work_items/617372#note_3734857083

References

Resolves https://gitlab.com/gitlab-org/gitlab/-/work_items/617372

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Modify the limit_snippets_created_by_user in lib/gitlab/application_rate_limiter/labkit_adapter/supported_rate_limits.rb limit to 1
• gdk restart
• The following request succeeds and creates a snippet:

curl --request POST "http://localhost:3000/api/v4/snippets" \
  --header "PRIVATE-TOKEN: $TOKEN" \
  --header 'Content-Type: application/json' \
  --data '{"title":"t","file_name":"a.txt","content":"x","visibility":"private"}'

• Repeat and get a 429 with {"message":{"error":"This endpoint has been requested too many times. Try again later."}}% response
• The project snippets endpoint returns the same response.

curl --request POST "http://localhost:3000/api/v4/projects/<PROJECT_ID>/snippets" \
  --header "PRIVATE-TOKEN: $TOKEN" \
  --header 'Content-Type: application/json' \
  --data '{"title":"t","file_name":"a.txt","content":"x","visibility":"private"}'

• A GraphQL request returns a 200 with "This endpoint has been requested too many times. Try again later." message.

curl --request POST "http://localhost:3000/api/graphql" \
  --header "Authorization: Bearer $TOKEN" \
  --header 'Content-Type: application/json' \
  --data '{"query":"mutation { createSnippet(input: {title: \"t\", visibility_level: private, blobActions: [{action: create, filePath: \"a.txt\", content: \"x\"}]}) { snippet { id } errors } }"}'

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