feat(tracing) Collect DSC for head of trace

[smeubank]

Describe the idea

Applications instrumented with a Sentry SDK should collect and propagate dynamic sampling context data to all outgoing requests and to Sentry.

Requirement:

When an application has an event which is the head of a trace,
the SDK shall collect and propagate all possible values in outgoing requests and to Sentry
so that contextual data can be propagated in all connected services across the continuation of trace

what should be sent?

baggage header

• sentry-trace_id
• sentry-public_key
• sentry-sample_rate
• sentry-release
• sentry-environment
• sentry-user_id
• sentry-user_segment
• sentry-transaction

Trace Envelope Header

• trace_id (string)
• public_key (string)
• sample_rate (string)
• release (string)
• environment (string)
• user_id (string)
• user_segment (string)
• transaction (string)

where should it be sent?

• in dynamic sample context (by baggage)
• in the envelope header
• meta data (was/is being done in JS to be check if still necessary)

Example:
Here is an example from the Sentry UI, where the implementation has already begun on the latest versions of the JS SDK. A request from the front end to a down stream applications includes in the request header baggage

Why do you think it's beneficial to most of the users

This ensures that further downstream applications connected in a distributed trace will receive all contextual data required for dynamically sampling.

Possible implementation

Implementation has already begun (at the time of this creation) on JS and Android. Refer to the SDK Developer documentation for further information

For full up to date requirements according to SDK Developer specifications please see the associated documentation.

Dynamic Sampling Context

Original PR for full context
Add spec for Dynamic Sampling Context #613