[xds]: export discoveryReq/Resp resource count, payload size metrics to help understand network bandwidth cost of control plane.

Title: export discoveryReq/Resp resource count, payload size metrics to help understand network bandwidth cost of control plane.

Description: export discoveryReq/Resp resource count, payload size metrics to help understand network bandwidth cost of control plane.

Metrics could be: Per request/response resource{added, removed} count, Raw protobuf config size

this way we can better understand the config plane network cost as well as data serialization/deserialization cost.

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