fix: honour 'usePathAsTransactionName' config var in Next.js instrumentation

[trentm]

Fixes: #3072

Checklist

• add a test case

[trentm]

Try this out via:

npm install 'elastic-apm-node@elastic/apm-agent-nodejs#trentm/nextjs-usePathAsTransactionName'

[apmmachine]

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Build stats

• Start Time: 2022-12-15T19:21:58.689+0000

• Duration: 25 min 56 sec

Test stats 🧪

Test	Results
Failed	0
Passed	320030
Skipped	0
Total	320030

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