feat(core): add sample rate for Vercel analytics and speed insights

[chanceaclark]

What/Why?

Adds an opinionated default on the sample rates for both Vercel Speed Insights and Vercel Analytics. Most of the time you don't need to capture 100%, especially on a heavily trafficked site.

For Vercel Analytics, there was a little lifting that was needed to be done. We don't necessarily want to send events 25% of the time, but rather 25% of the users get analytics tracked per session.

Testing