feat(rum): add sessionReplayOnErrorSampleRate - #24

Motivation

A session is stored, and charged for, as a whole. That makes "keep every error, sample everything else" awkward to express by filtering events: dropping individual events still leaves the session stored, so the thing that actually counts does not go down.

sessionReplayOnErrorSampleRate expresses it where it counts. A session drawn by this rate records a replay from the start but uploads nothing. If it never reports an error, nothing is ever sent and the session is never stored. On the first error the withheld recording is released, so the replay covers what led up to the error rather than starting at it.

Changes

• New init option sessionReplayOnErrorSampleRate, drawn only for sessions the plain sessionReplaySampleRate draw missed, so a session is never counted by both rates. It adds one tracking type and a BUFFERED_ON_ERROR replay state.
• startSessionReplayRecordingManually now defaults to false when either replay rate is set: a session that has to be recording before the error cannot wait to be started by hand.
• Segment collection withholds segments while the session has not errored. The five second rotation keeps the segment growing instead of cutting it, and a buffer spanning more than a minute is dropped and restarted from a fresh full snapshot — so what is eventually released stays a minute at most and can be played from its own start.
• The existing segment byte limit still applies while withheld, and restarts are spaced out, so a document whose full snapshot alone exceeds that limit degrades instead of restarting in a loop.
• A withheld buffer belongs to the session that produced it. It is released only when that same session reports the error; if the session expires or is renewed first the records are dropped, so an expiry can never turn into an upload for a session that never errored. A page being merely hidden keeps the buffer, since it comes back and the full snapshot would otherwise be lost with it.
• Dropped buffers roll their replay stats back, and neither has_replay nor the replay link is offered while a replay is withheld, so nothing points at a replay that does not exist.
• Errors the SDK raises about its own transport (error.source === 'agent') release nothing: an unreachable intake should not turn every session into an error session.
• Telemetry on release reports how much history the buffer actually carried, so the window can be checked against the one it promises.

Also included: the profiling test helpers imported from package names this repository does not publish. Because the helper is re-exported from the rum test barrel, those imports stopped roughly 220 specs from loading at all; fixing them brings the suite back.

Test instructions

yarn test:unit. New specs cover withholding, release, the checkout window, the byte limit, the stats rollback, hidden pages, and session expiry and renewal.

By hand, against yarn dev:

1. init with sessionSampleRate: 100, sessionReplaySampleRate: 0, sessionReplayOnErrorSampleRate: 100, then call startSessionReplayRecording().
2. Interact with the page for a while and watch the network panel: no replay request is made.
3. Report an error. One replay request follows, and its segment metadata has has_full_snapshot: true with records reaching back before the error.
4. Repeat with the session running past a minute before the error: the released segment reaches back at most a minute, not the whole session.

Driven through Playwright against the same page, a 108 second session uploaded nothing before the error and then released a segment reaching back 48 seconds, with the full snapshot present.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.