Prometheus Remote Write exporter names can have consecutive underscores

[Seefooo]

    [One more requirement](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#prometheus-compatibility:~:text=the%20_%20character.-,Multiple%20consecutive%20_%20characters%20MUST%20be%20replaced%20with%20a%20single%20_%20character.,-The%20Unit%20of). Feel free to open an issue and either do in a follow up PR or I can mark it up for grabs.

Originally posted by @aabmass in #1359 (comment)

The names & labels after going through string sanitization can have consecutive underscores. This can be fixed, but may require some restructure of the code to not clobber the __name__ label.