fix(tracing-internal): Prefer fetch init headers over fetch input headers

[Lms24]

When a fetch call is made with a Request object as the input/resource param (i.e. the first) and additionally provided headers in the fetch init options (the second param),

const request = new Request('http://example.com/api/test/', {
  headers: { foo: '11' },
});

fetch(request, {
  headers: { bar: '22' },
});

browsers ignore the headers from the request object but only add the headers from the init options when making the request.

Our fetch instrumentation had it the other way around, preferring headers from the request object over the headers in the options, as reported in #10172 . This PR fixes this behaviour by essentially turning around the logic we previously had.

fixes #10172

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.37 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.02 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.4 KB (+0.02% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	30.99 KB (0%)
@sentry/browser - Webpack (gzipped)	22.33 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	74.69 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	66.33 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.16 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	23.96 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	208.94 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	96.99 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	71.59 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	35.17 KB (+0.02% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.75 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped)	22.37 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.41 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.52 KB (+0.01% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17 KB (0%)

[lforst]

nice