add instrumentation for read_multi

[ignatiusreza]

Summary

add instrumentation for read_multi as currently it's not possible to know what the hit rates are from read_multi

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @rafaelfranca (or someone else) soon.

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[maclover7]

👍 nice! Should we add a CHANGELOG.md entry as well?

[ignatiusreza]

CHANGELOG.md updated 👍

[ignatiusreza]

Rebased to resolve conflict with master on CHANGELOG.md.. anything else I could do to help move this forward?