Allow sending the subgraph.graphql.document attribute to Studio via OTLP

[timbotnik]

There is valuable information in OTel traces that is not being passed to Studio, specifically the GraphQL document being sent to a subgraph. Today, we filter this attribute out from the Studio path.

After this change, if a user is using the “recommended” attribute config or enables this with a specific attribute configuration, Router would allow the subgraph document to flow through to traces sent to Studio. e.g.

telemetry:
  instrumentation:
    spans:
      default_attribute_requirement_level: recommended

or

telemetry:
  instrumentation:
    spans:
      subgraph:
        attributes:
          subgraph.graphql.document: true

We should decide if this is something that would make sense to enable. There are potential downsides:

• More data will be sent on the wire to Studio to capture these GraphQL document payloads.
• Potentially PII could be included in any inline variables

I'd feel less concerned about this if it was:

• Fully opt-in rather than being included with our "recommended" options
• Using the normalized operation string instead of the raw document

Fixes #issue_number

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