Allow `registerOTel` to either return created resource or pass custom resource

[kzuraw]

Problem

I recently was working on implementing custom OTEL metrics in our Next.js app (using app router) and I needed to create MeterProvider from @opentelemetry/sdk-metrics. Problem with that provider is it requires @opentelemetry/resources Resource to be defined. As a quick fix I recreated resource from scratch.

Solution

Instead of recreating OTEL resource @vercel/otel can either return created resource that I could pass into MeterProvider or allow me to provide my own resource so both MeterProvider & registerOTel will use the same object.