Move the onFinishCallback before the span is finished

[stefanosiano]

Description

#3219 (comment)

[markushi]

Let's also keep our SpanFrameMetricsCollector in mind, as it's onSpanFinished callback requires the finish date to be already set.

[markushi]

Given that we have a mixed behavior for txn/spans, let's consider this broken and fix it.
Let's apply this to both Spans and Transactions.