feat: option for propagationHeaders which are used in fetch patch

[lemusthelroy]

This allows us to manually pass headers that we wish to propagate. E.g. headers from a Netlify Request in a serverless function. These headers can then be used in the fetch instrumentation to ensure we can forward on the trace context.

E.g.

const fetchInst = new FetchInstrumentation()

await createTracerProvider({
    // ...props
    propagationHeaders: netlifyRequest.Headers,
    instrumentations: [
      fetchInst
    ],
  })

[lemusthelroy]

not totally convinced this is the best way to make this forwarder accessible to the fetch instrumentation so may rework this

[eduardoboucas]

@lemusthelroy Can you add a brief description that describes what problem this tries to solve and how it's meant to be used, so that it's easier to review?

[lemusthelroy]

@lemusthelroy Can you add a brief description that describes what problem this tries to solve and how it's meant to be used, so that it's easier to review?

Will do, sorry had been so focused on the code