[Yarpc-Go] Implement Metrics Tag Decorator

[sakshi4016]

• [x] Description and context for reviewers: one partner, one stranger This PR introduces a Metric Tag Decorator interface that allows service owners to add custom tags to the inbound and outbound metrics emitted by YARPC-Go. The main motivation for this change is to support adding a custom tenancy tag to YARPC metrics. To prevent high cardinality issues, this tag is opt-in. By default, the tenancy tag is set to dropped, but service owners can override it by providing implementation of the TenancyTagDecorator interface.

• [x] Docs (package doc) ERD : https://docs.google.com/document/d/1ceTSRSYCaRO5yDXQ6FFgyfloxrIHWrD6_CXVWdi00VU/edit?usp=sharing

RELEASE NOTES:

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