feat: Tracing without Performance

[cleptric]

TL;DR

These changes will kick off a movement where the performance feature is no longer required to implement distributed tracing.

At its core is a new PropagationContext, an object holding all required properties to implement distributed tracing in Sentry: a trace ID, span ID, parent span ID, and a Dynamic Sampling Context.

The PropagationContext is attached to each scope and is used as a fallback if tracing is not enabled (Options::isTracingEnabled() is false).

New API

Naming is not final!

Sentry\continueTrace

A new top-level function to continue a trace. This function is context-aware, as in it will either mutate the PropagationContext on the scope or return a populated TransactionContext if tracing is enabled, based on the values of a passed in sentry-trace and baggage header.
While this is not a great API, we have to maintain BC and are willing to live with this for now.

Sentry\traceparent

A new top-level function to receive the trace context to be used as an HTML meta tag for SSR applications or as a header value to be attached to outgoing HTTP requests. This function is context-aware, as in it will either return the trace context of the current span if tracing is enabled or fall back to this value from the PropagationContext stored on the scope.

Sentry\baggage

A new top-level function to receive the Dynamic Sampling Context to be used as an HTML meta tag for SSR applications or as a header value to be attached to outgoing HTTP requests. This function is context-aware, as in it will either return the Dynamic Sampling Context of the current transaction if tracing is enabled or fall back to this value from the PropagationContext stored on the scope.

Each event always contains a trace context

If tracing is disabled, we will now use the PropagationContext as a fallback to attach a trace context to each event, in the case of this SDK, to all error, transaction, and check-in events.

Each event always contains a Dynamic Sampling Context (trace) envelope header item

As this SDK still uses the /store endpoint if tracing is disabled, we can only attach the DSC to check-in events in this case.

DynamicSamplingContext::fromOptions

To propagate a Dynamic Sampling Context in case tracing is disabled, a new method was added that constructs the Dynamic Sampling Context from the current options of the client.

Part of #1525

[stayallive]

I have a hard time finding any wrong coce-wise. Looks good!

Just a single TODO we (might) need to address but otherwise good to go!