Description

We should provide a way for users to make a sampling decision for a trace based on the sampling decision of the previous trace (i.e. the one we link since #14992). Concretely, we propose to add an option to tracesSampler:

Sentry.init({
  dsn: '...',
  tracesSampler(({previousTraceSampled}) => {
    if (previousTraceSampled) {
      return 1.0; // could also just increase the rate, e.g. to 0.5
    } else if (previousTraceSampled === false) {
      return 0;
    }
    return 0.05;
  })
})

where previousTraceSampled is typed as boolean | undefined. The semantics for all values:

• true - previous trace was positively sampled and sent to Sentry (this makes no guarantees that this trace in fact was stored; it can still be dropped by Relay)
• false - previous trace was negatively sampled and not Sent to Sentry
• undefined - multiple implications
  • the current trace is the first one
  • previous trace collection is disabled by users
  • previous trace collection is not available (e.g. server SDK)

This allows users to ensure that a trace chain is longer or in general more complete.

Important notes:

• By default the SDK will not continue a positive sampling decision based on the previous trace. This must be a concious user decision to opt into.
• The potential quota increase must be mentioned in JSDoc and docs