This would allow developers to create custom transactions and spans in their code to capture information about their application's performance and behavior.

At the very least support:

• Transaction
• Span
• startTransaction()
• startChild()
• getSpan()
• tracesSampleRate option
• tracesSampler option

Align implementation with:

• https://docs.sentry.io/platforms/java/performance/instrumentation/custom-instrumentation/
• https://docs.sentry.io/platforms/apple/guides/ios/performance/instrumentation/custom-instrumentation/