ReactRouter v6 instrumentation does not consider `initialEntries` for pageload transactions when using memory router

[lforst]

Description

A user reported that they are using withSentryReactRouterV6Routing(Routes) with createMemoryRouter().
The user is passing in the initial routes to render via createMemoryRouter's initialEntries option.

They observed that for their application they exclusively got /index.html pageload transactions when the app loads. This is undesired behavior.

We should probably use the initialEntries option for pageload transactions instead of window.location.pathname when available.

One thing we need to be somewhat wary of is that people may use the same opts for a memory router and for a browser router and for the browser router it would probably be undesired to take the initialEntries. Maybe we need to be clever to properly distinguish.

This should land in both the v8 and develop branches.

[lforst]

@onurtemizkan is this something you would want to work on with medium-high-ish priority?

[onurtemizkan]

@lforst - Yes sure 👍