autoexport: Change WithFallback options signatures

[pellared]

Problem Statement

The current definition of fallback options is as following:

func WithFallbackMetricReader(exporter metric.Reader) MetricOption

The problem is that we need to create an instance of a metric reader or span exporter and it may not even be needed/used (it may even leak resources).

Proposed Solution

Change the signature to accept a factory function e.g.

func WithFallbackMetricReader(factory func(context.Context) (metric.Reader, error)) MetricOption

[hcelaloner]

Hi @pellared, I went ahead and prepared a PR for the requested change: #4891

I directly changed the signature of the method as requested. Doesn't this create a breaking change for existing autoexport users who use that method? How do you proceed in that kind of situation in opentelemetry-go-contrib? Should we create a new method and deprecate the old one instead?