function startTransaction(context, customSamplingContext) {
    return callOnHub('startTransaction', __assign$4({}, context), customSamplingContext);
}
Starts a new `Transaction` and returns it. This is the entry point to manual tracing instrumentation.

A tree structure can be built by adding child spans to the transaction, and child spans to other spans. To start a
new child span within the transaction or any span, call the respective `.startChild()` method.

Every child span must be finished before the transaction is finished, otherwise the unfinished spans are discarded.

The transaction must be finished with a call to its `.finish()` method, at which point the transaction with all its
finished child spans will be sent to Sentry.

@param context Properties of the new `Transaction`.
@param customSamplingContext Information given to the transaction sampling function (along with context-dependent
default values). See {@link Options.tracesSampler}.

@returns The transaction which was just started