Add ability to reference $config in header values

[pubmodmatt]

Add the ability to reference $config variables in headers sent to connectors. This allows, for example, specifying a static token in an authorization header.

Config YAML:

preview_connectors:
  subgraphs:
    subgraph_name:
      $config:
        api_token: ${env.API_TOKEN}

GraphQL Schema:

@source(
    name: "jsonPlaceholder"
    http: {
      baseURL: "https://jsonPlaceholder.typicode.com/"
      headers: [
        { name: "Authorization", value: "Bearer {$config.api_token}" }
      ]
    }
  )

Note that curly brace characters { and } are technically allowed within header values. Currently, since the variable reference grammar implemented here only accepts names starting with the $confg namespace, this should not cause a problem unless the customer actually wants to send a literal value containing the string {$config, which is unlikely. However, if the grammar is expanded in the future, this may become a concern - one possible solution would be to allow escaping the curly brace characters with something like {{ and }}.

---

Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[router-perf]

CI performance tests

• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload

[lennyburdette]

one comment, otherwise looks pretty awesome!