Add support for wide event sampling

[lmac012]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1205648422731273/task/1213138183491587?focus=true
API proposal: https://app.asana.com/1/137249556945/project/1202552961248957/task/1213453461866489?focus=true

Description

Steps to test this PR

• Build and install from the base branch
• Launch the app
• Disable internet connection (e.g. turn airplane mode)
• Perform an action that creates a wide event (e.g. use fire button)
• Inspect wide_events.db and verify that the wide event is stored
• Update the app using build from this branch
• Inspect wide_events.db and verify that wide_events table now has sampling_probability and all existing records have it set to 1.0.
• Enable internet connection
• Verify that the event is sent with sample_rate = 1.0 param.

No UI changes

---

Note

Medium Risk
Touches statistics persistence, migration, and outbound telemetry shape; default 1.0 preserves existing behavior until features opt into lower rates.

Overview
Adds optional sampling to wide events so high-volume flows can record a fraction of journeys without call-site handling for dropped flows.

WideEventClient.flowStart now accepts samplingProbability (0.0–1.0, default 1.0). At start, a random draw decides whether to persist the flow; sampled-out runs return SAMPLED_OUT_FLOW_ID and all later flowStep / flowFinish / interval / abort calls on that ID are no-ops with no DB writes. Kept flows store the probability on the row (Room v3 migration defaults legacy events to 1.0).

API and analytics payloads now emit the stored rate instead of a fixed value: global.sample_rate on the wide-event POST body and global.sample_rate on pixels. Cursor wide-events docs describe the parameter and sentinel behavior.

Tests across app, VPN, PIR, subscriptions, and sync are updated for the new flowStart arity; WideEventClientTest covers validation, sampling, and sentinel no-ops.

^{Reviewed by Cursor Bugbot for commit 09d95d0. Bugbot is set up for automated code reviews on this repo. Configure here.}

[lmac012]

• Add support for wide event sampling #7965 👈 (View in Graphite)
• develop

This stack of pull requests is managed by Graphite. Learn more about stacking.

[github-actions]

⚠️ API Proposal reminder

This PR changes the public surface of one or more -api modules, but the API Proposals field of the PR description is empty. This is a non-blocking reminder — please double-check whether a proposal is needed.

Changed -api source files:

• statistics/statistics-api/src/main/java/com/duckduckgo/app/statistics/wideevents/WideEventClient.kt

To clear this reminder, edit the PR description and either:

• add the approved API Proposal link(s) in the API Proposals field (one per line), or
• write None there if this PR does not change the public API (e.g. KDoc, comments, tests).

See the API Proposals section in .cursor/rules/contributions.mdc for details.

[joshliebe]

Code makes sense and tested, works as expected ✅