fix(nextjs): Enable fetch span when OTel setup is skipped

[s1gr1d]

Next.js has their own fetch instrumentation, but when people use their own OTel setup, we don't want to disable that.

fixes #17581

[chargome]

@s1gr1d did you test this in a sample app? Might be worth adding a minimal e2e for this scenario at some point.

[s1gr1d]

Yes I tested it in the reproduction that was provided in the linked issue. Will create an E2E test for it!

I also removed the check for hasSpansEnabled because this would only lead to a lot of non-recording spans when tracing is not enabled. We only want to check for skipOpenTelemetrySetup.

[github-actions]

node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	8,808	-	9,143	-4%
GET With Sentry	1,365	15%	1,394	-2%
GET With Sentry (error only)	5,983	68%	6,056	-1%
POST Baseline	1,186	-	1,199	-1%
POST With Sentry	496	42%	531	-7%
POST With Sentry (error only)	1,048	88%	1,065	-2%
MYSQL Baseline	3,259	-	3,318	-2%
MYSQL With Sentry	407	12%	453	-10%
MYSQL With Sentry (error only)	2,648	81%	2,670	-1%

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