Ensure existing headers are preserved for fetch requests

[yousif-bugsnag]

Goal

Fixes a bug in the fetch request tracker where any headers that are set in a Request object passed to fetch were not being preserved. Similarly if headers are set using the Headers class in the request options these would also be wiped.

This is because trace propagation headers were always being merged with the options object (the second argument passed to fetch) as an object literal, however request headers can be set in a few different ways:

• As an object literal in options
• As a Headers instance in options
• In a Request object passed as the first argument to fetch

The behaviour of fetch is such that if headers are set in the options object these will take precedence, and any headers defined in a Request object are ignored.

Design

Reworked the mergeRequestHeaders logic to handle all of the cases listed above, with headers defined in fetch options taking precedence.

Testing

Added additional unit test cases and e2e scenarios

[github-actions[bot]]

Browser bundle size

NPM build

	Package
Before	183.98 kB
After	183.98 kB
±	No change

CDN build

	Unminified	Minfied	Minified + gzipped
Before	91.76 kB	34.46 kB	10.41 kB
After	92.67 kB	34.77 kB	10.48 kB
±	+905 bytes ⚠️	+312 bytes ⚠️	+74 bytes ⚠️

Code coverage

Ok	File (✨=New File)	Lines	Branches	Functions	Statements
🔴	/home/runner/work/bugsnag-js-performance/bugsnag-js-performance/packages/request-tracker/lib/request-tracker-fetch.ts	73.58% (-11.26%)	54.9% (-2.67%)	55% (-15%)	60.56% (-9.2%)

Total:

Lines	Branches	Functions	Statements
87.58%(-0.39%)	78.62%(-0.83%)	88.14%(-1.1%)	84.21%(-0.57%)

Generated against 3de148a55a2382345a82bbecd6d535b88cf1ca62 on 12 April 2024 at 11:39:39 UTC