Updated discovery.proto. Removed optional label.

[auyidi1]

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[kate-goldenring]

@auyidi1 thank you bringing this up as some libraries no longer allow optional fields unless a flag is set.

This change should also come with a rebuild of the generated code (cd discovery-utils && make build) and dependent changes to the discovery handler implementations. I know this is a delayed review. Let me know if you would like me to push those changes into here or whether you are happy to take this on.