fix(react): Add POP guard for long-running pageload spans

[onurtemizkan]

This resolves the issue that occurs when an extra navigation transaction is created after a prematurely ended pageload transaction in React Router lazy routes.

This apparently occurs when there's a long-running pageload with lazy-routes (after fetching assets, there are multiple potentially long-running API calls happening).

This causes the pageload transaction to prematurely end, even before the fully parameterized transaction name is resolved. The reason is that there can be a POP event emitted, which we subscribe to create a navigation transaction. This ends the ongoing pageload transaction before its name is updated with a resolved parameterized route path, and starts a navigation transaction, which contains the remaining spans that were supposed to be a part of the pageload transaction.

This fix makes sure the initial POP events are not necessarily treated as navigation pointers, which should fix both:

• Duplicate / extra navigation transactions having a part of pageload spans.
• Remaining wildcards in the pageload transaction names

[chargome]

Nice find! Any way we can test this in e2e?