feat: Add "STRING" to the list of valid metric units in golden metrics schema

[shivalagisetty]

Relevant information

Story : https://new-relic.atlassian.net/browse/NR-328461

Adding CLS to the golden signals and summary signals. As part of this I am adding STRING to golden signals allow list

Checklist

• [ ] I've read the guidelines and understand the acceptance criteria.
• [ ] The value of the attribute marked as identifier will be unique and valid.
• [ ] I've confirmed that my entity type wasn't already defined. If it is I'm providing an explanation above.

[CLAassistant]

All committers have signed the CLA.

[otaviocarvalho]

Other changes are blocking this one to proceed: link

[albertoolivan]

After 9 months without need it, we decide to close it