Allow metrics with the same name (Option #2)

[jaydeluca]

Alternate approach to #1728

I still also have scrape time validation occurring along with registration time, but we can remove that if we think the registration time validation is sufficient protection.

[zeitlinger]

I still also have scrape time validation occurring along with registration time, but we can remove that if we think the registration time validation is sufficient protection.

I would opt for registration only.

What does that mean for clients that only work on snapshots like the OTel SDK?

https://github.com/open-telemetry/opentelemetry-java/blob/da310cc1fcd06e606426649f6c8db96958d9bb12/exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/PrometheusMetricReader.java#L104