Initial cost metrics

[BrynCooke]

This PR adds selector that will be used for displaying cost metrics on the router.

A separate PR will be raised for query depth and height under a different category.

telemetry:
  instrumentation:
    instruments:
      supergraph:
        cost.actual: true
        cost.delta: true
        cost.estimated: true

In addition the following selectors are introduced:

• cost.estimated
• cost.actual
• cost.delta
• cost.result

This allow you to attach cost related data to spans instruments or events.

There is a lifecycle issue present in this PR which prevents cost.actual and cost delta from functioning correct that we will tackled separately. This is because supergraph response happens before the response payload has been returned.

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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

Note any exceptions here

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. ↩