fix(tracing): Set correct parent span id on fetch sentry-trace header

[AbhiPrasad]

In slack we reported a bug where there seemed to be the wrong parent span id attached to outgoing sentry-trace headers: https://sentry.slack.com/archives/CDXAKMGTU/p1690830621077679

This ends up being because the addTracingHeadersToFetchRequest attempts to use the transaction instead of the active request span to generate the outgoing sentry-trace header.

This fixes that by adding the span to the addTracingHeadersToFetchRequest method.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.1 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.41 KB (+0.03% 🔺)
@sentry/browser - Webpack (gzipped)	21.95 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.83 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.29 KB (+0.04% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.33 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	221.03 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	86 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	60.48 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	30.41 KB (+0.01% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.16 KB (0%)
@sentry/react - Webpack (gzipped)	21.96 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	92.93 KB (-0.01% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	50.89 KB (+0.02% 🔺)