Client instrumentation: onRouterTransitionStart (#77791)
Adds a new API for observing the start of an App Router navigation:
```ts
// <PROJECT_ROOT>/instrumentation-client.ts
export function onRouterTransitionStart(
url: string,
navigationType: 'push' | 'replace' | 'traverse'
) {
// ...
}
```
`navigationType` is one of "push", "replace", or "traverse". This is
inspired by the Navigation API:
https://developer.mozilla.org/en-US/docs/Web/API/NavigateEvent/navigationType
`onRouterTransitionStart` is intended for sending logs to a performance
monitoring tool. It is _not_ intended as a general purpose event for
implementing application behavior.
It fires at the start of every client-side navigation, including those
initiated by a popstate (back/forward) event.
There is no corresponding API for observing the end of a navigation. We
intend to build support for this in the future, but it's a non-trivial
problem space due of the streaming nature of React transitions. Refer to
the React Transition Tracing proposal for more details:
https://github.com/reactjs/rfcs/blob/transition-tracing/text/0235-transition-tracing.md
`onRouterTransitionStart` is only called during App Router navigations.
To instrument Pages Router navigations, use `router.events`.
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