Avoid instrumenting Next.js telemetry requests (#97182)
Stacked on #96914.
## Summary
- Keep Next.js telemetry uploads out of application fetch activity by
bypassing the Next.js patched-fetch wrapper.
- Preserve user-provided fetch implementations and ignore unbranded or
non-callable original-fetch properties.
## Verification
- All six telemetry transport tests passed, covering branded, unbranded
and custom fetch implementations.
Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Will Binns-Smith <755844+wbinnssmith@users.noreply.github.com>
