This is a new callback introduced in DiagnosticSource 4.6.0 that instrumentations should call when it read headers and set parents/context on the Activity but before Activity is started.
This serves two purposes
tracing tool can support custom headers in the absence of default ones without creating a new Activity
tracing tool can make sampling decision earlier and prevent starting activity if it's sampled out
This is a new callback introduced in DiagnosticSource 4.6.0 that instrumentations should call when it read headers and set parents/context on the Activity but before Activity is started.
This serves two purposes