[identity] Add span tracing to grpc handlers

Summary:
Adds span tracing to the grpc handlers through the instrument attribute

Should hopefully give us more information to diagnose errors like https://linear.app/comm/issue/ENG-7866/getkeyserverkeys-error-on-staging-identity-unexpected-error#comment-94f5f650

Depends on D11824

Test Plan: cargo check

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11825