Fix parts of request metadata being missing from some events

[imjoehaines]

Goal

Request metadata is stored in two places on an Event — some goes in event.request and other bits are added as metadata. This is because the Bugsnag API accepts some request data as a top-level field, but we add more data than the API supports so these bits have to go in metadata

We were adding this metadata on the requestClient instead of the event and were doing so in an on error callback, which is too late to affect the already-created event. This would affect any subsequent events, though, so the data was present in some cases. This was broken when we moved from adding request data when our middleware was called to adding it in a callback, which was done to ensure we reported the final state of the request as other middleware could mutate it

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	43.19 kB	13.21 kB
After	43.19 kB	13.21 kB
±	No change	No change

code coverage diff

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