perf(node): Short circuit flushing on Vercel only for Vercel

[lforst]

Only adds the HttpInstrumentation that is supposed to flush for Vercel when we are on Vercel. Should reduce some overhead.

Also adds a bit more explanation because of #15602 (comment)

[mydea]

This is good 👍
Maybe a follow up to this - could we only add this instrumentation if needed (e.g. when process.env.VERCEL is set)? THis would avoid this (slight, but still) overhead for all other platforms 🤔

[lforst]

Maybe a follow up to this - could we only add this instrumentation if needed (e.g. when process.env.VERCEL is set)? THis would avoid this (slight, but still) overhead for all other platforms 🤔

Actually, let me do this in this PR directly 👍 separation of concern wise I don't like it too much but for now it makes sense.