[DSC] include tracesSampleRate in Dynamic Sampling Context

[Lms24]

We want to include the sample rate with which an event was sampled in the dynamic sampling context (DSC) which we send to Relay in the trace envelope header and propagate to downstream SDKs via the baggage header.

Problem:
As customer adopt DS with in product Sentry server side traces and transaction sample rate rules, then they should change the sample rate in their applications. In the situations where there is distributed tracing, there could be conflicting sample rates on different projects, leading to issues where we cannot provide DS

where should it be sent?

• in dynamic sample context (by baggage)
• in the envelope header
• meta data (today becomes not necessary)

Does what we sent in the envelope header need to be same as http header (baggage)?

• Yes

where does the public key need to be?

• in the DSC, meaning it should be sent in the envelope header and in the propagation mechanism (i.e. baggage http header)

[smeubank]

magic sample rate

• any BE calls will be upgrade to 100%
• if the SDK does not forward properly then it will not know the origin SDK
• so we should not recommend upgrading unless all your BE services are upgraded to a proper SDK version which propagates and forwards the new header context data correctly
• why would BE go to 100%
• because client has set 1

JS SDK status

• need sampleRate and public key from where it started (head of trace)
  • include in payload
  • & in envelope header (baggage)
  • meta data (today becomes not necessary)?

Traces:

• sampe rate
• public key (head of trace)
• environment (done)
• release (done)
• transaction (send unparameterized for now)
  • needs clear documentation around possible limitations per platform specific
  • example (parameterized transaction express may not come till the end)
  • can we solve in product?
    • regex
  • browser prio
    • examplereact - we can never get the parameterized at beginning of transaction
      • starts as index.html and turns into /api/id/route
• user
• usersegment

Transaction

• this is NOT in any SDK today
• this is mandatory
  • must be in baggage trace-context only exist per that transaction
• if in JS then we have browser and node
• then prio Python and Java, other server-side platforms

Problem

• Need down stream services to propagate baggage properly, before upstream (browser)
• include sample rate then in Sentry we know

Solution:

• Users go forward with DS but don't change their sampleRate
• concerns relative to how often requests from a FE go to different downstream services

[smeubank]

We'll include public key and transaction in one PR for this issue, user, and usersegment

[Lms24]

To be more precise, we will include public key and sample rate, as well as transaction, user id and user segment in one PR (#5292)