fix: plugin metrics sdk used to do some undefined behavior

Summary

• fix: plugin metrics sdk used to do some undefined behavior

Previous implementation would call functions over the ffi boundary which weren't defined with the required extern specifiers, which is not allowed.

• fix: reduce size of FfiRpcHandler struct

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• Close #993 (closed), #828 (closed)
• Cherry-pick to: none
• Docs follow-up: not necessary