DIGIT-2819 Add Rack Stack span and normalize Queue Time span type/subtype

[gustoliveira]

• Add a Rack Stack span around the Rack middleware pipeline and end it when the first “real work” event starts (Rails controller or Grape endpoint) or when GraphQL parsing begins.
• Change Queue Time span to app with custom subtype/action and guard against nil spans when sampling or max spans are disabled.
• Track the Rack Stack span on the transaction object and register custom subtypes in the span type registry for validation.

[github-actions]

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