Record span events for fetch network events

[bripkens]

Why

It can be really helpful to understand what contributes to fetch request times:

• DNS lookup?
• Connection establishment/TLS?
• TTFB?

What

The official OTel fetch instrumentation for the web has the capability to
attach network events to fetch spans. It would be really nice if that
would be supported by @vercel/otel too:

https://github.com/open-telemetry/opentelemetry-js/blob/01cea7caeb130142cc017f77ea74834a35d0e8d6/experimental/packages/opentelemetry-instrumentation-fetch/src/fetch.ts#L112C11-L112C31