Expand description
Gathering what one request did, and only that request.
The instrumentation bus is global: every query in the process dispatches to the same subscribers. A debug bar needs the opposite — the queries this request made, so the number on the page belongs to the page.
A task-local does that. Every request is a task, everything the handler
awaits stays on it, and so a collector installed for the task sees that
request’s events and nobody else’s. Work moved off the task with
tokio::spawn is not collected, which is the honest limit of the approach
and is stated on the bar rather than hidden.