Implement Sampling Seed Propagation

[kahest]

Description

See https://github.com/getsentry/projects/issues/316

To increase the chance of capturing complete traces when users return a new sample rate tracesSampler in backend services, we propagate the random value used by the SDK for computing the sampling decision instead of creating a new random value in every service. Therefore, across a trace every SDK uses the same random value.

Develop: Spec

See also

Sample implemeantion in PHP getsentry/sentry-php#1793.

[philprime]

I understand that we neeed to add the sample_rate header to the baggage, but do we also need to implement this "[..] the sampling decision (trailing -1 or -0 from the sentry-trace header) are present in the incoming trace [..]`?

We already send a header sampled in the baggage.

[philprime]

Update from my side: Looking at the encoding logic in SentryTraceHeader.m, it seems like the value sampled is already appended to the header string here:

sentry-cocoa/Sources/Sentry/SentryTraceHeader.m

Lines 25 to 31 in ba3679b

	- (NSString *)value
	{
	return _sampled != kSentrySampleDecisionUndecided
	? [NSString stringWithFormat:@"%@-%@-%i", _traceId.sentryIdString,
	_spanId.sentrySpanIdString, _sampled == kSentrySampleDecisionYes ? 1 : 0]
	: [NSString stringWithFormat:@"%@-%@", _traceId.sentryIdString, _spanId.sentrySpanIdString];
	}