otelconf: Increase test coverage for Prometheus-related code

Any tests that could be added?

i recall running into issues w/ ipv6 support in the collector when i was manually testing this previously.

_Originally posted by @codeboten in https://github.com/open-telemetry/opentelemetry-go-contrib/pull/6815#pullrequestreview-2628187338_
            