fix(nextjs): Stop injecting sentry into API middleware

[lobsterkatie]

During the build of a nextjs app, our SDK injects the user's sentry.server.config.js (which contains the call to Sentry.init()) into all API routes, so that withSentry will work. That said, we don't yet support nextjs middleware, so we shouldn't be injecting the file into middleware pages, even if they're in the api/ folder.

This prevents that by adding another condition to shouldAddSentryToEntryPoint.

Note: This came up as part of the issue referenced below, and it does fix it, but only in a temporary way. The underlying issue is (I believe) something outside of sentry. I've encouraged the user to debug it, so that when we start supporting middleware (and therefore remove this filter), he doesn't run into problems again. See this comment.

Fixes #4507.

[github-actions]

size-limit report

Path	Base Size (52520ba)	Current Size	Change
@sentry/browser - CDN Bundle (gzipped)	19.69 KB	19.69 KB	+0.01% 🔺
@sentry/browser - CDN Bundle (minified)	62.91 KB	62.91 KB	+0.01% 🔺
@sentry/browser - Webpack	22.21 KB	22.21 KB	0%
@sentry/browser - Webpack - gzip = false	76.02 KB	76.02 KB	0%
@sentry/react - Webpack	22.24 KB	22.24 KB	0%
@sentry/nextjs Client - Webpack	46.31 KB	46.31 KB	0%
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.22 KB	28.23 KB	+0.02% 🔺

[klaasman]

It's an unlikely situation, but in theory a user could name its non-middleware fn something like 'pages/api/_middleware/foo.ts', which will match this check and thus inject sentry where it shouldn't.