add requestContentLength breadcrumb metadata for xhr and fetch requests

[yousif-bugsnag]

Goal

Adds requestContentLength metadata to network breadcrumbs for XHR and fetch requests. This is the length of the request body in bytes.

Design

If a body is passed to XMLHTTPRequest.send() or fetch() the length in bytes of the body is calculated and recorded prior to calling into the respective native methods.

The content length is not calculated if the request body is a ReadableStream as these cannot be read without interfering with the successful completion of request.

The content length is also omitted if the request body is FormData or Document object as it's difficult to calculate accurately for these object types.

Where possible the byte length of the request body is read directly, (e.g. for Blob, TypedArray, ArrayBuffer and DataView objects). Otherwise the request body is serialized and a Blob is used to get the byte length of the UTF-8 encoded body.

Testing

Manually tested that the calculated content length matches the Content-Length header for the various request types.

Since the unit tests run in Node and not in the browser, this is covered by e2e tests. New e2e tests have been added to check that the content length is calculated for the various request types.

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	44.11 kB	13.47 kB
After	44.63 kB	13.64 kB
±	⚠️ +524 bytes	⚠️ +161 bytes

code coverage diff

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Generated by 🚫 dangerJS against 7b71722

[gingerbenw]

Forgive me, but what does reflect do?