Docs: Clarify that custom span attributes are part of an enterprise plan

[lleadbet]

Describe the solution you'd like

There is a small footnote here: https://www.apollographql.com/docs/router/configuration/telemetry/overview#router-telemetry-attributes

and in the text here: https://www.apollographql.com/docs/router/configuration/telemetry/instrumentation/spans/#attributes

But isn't made abundantly clear that this is paid functionality. A bolder callout could help ensure that users don't use this and are frustrated that they need a license to use it.

Describe alternatives you've considered

None.

[BrynCooke]

@shorgi Can you take a look at this?