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]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[niltonvasques]

Is this the time between the request leaves nginx and starts to be process by rails, right? This probably is also the time that the request lays on puma queue, not only the rack stack.

[gustoliveira]

Actually, queue-time measure nginx+puma queue, while rack-stack only starts after puma dequeue, the correct flow bellow:

enter nginx -> start queue-time -> nginx route to puma -> puma queue -> puma processing -> puma dequeue -> stop queue-time -> start rack-stack -> call rack -> ...rack middlewares -> stop rack-stack -> start GrapqhController span -> start GraphqlController#execute span -> ...

The previous subtype descriptions left room for misinterpretation, so I updated them to make this distinction explicit (and clarified queue-time as well)