Drop RpcProxy usage from metering_agent

Drop one more use of the RpcProxy compatibility class in favor of
direct use of oslo.messaging APIs.

Part of blueprint drop-rpc-compat.

Change-Id: I3c68aa246d88447f793aa9890f10f5043a41338d
Updated from global requirements

Change-Id: I74345f78a169282b99ed3575f19afa3ca0548839
Merge "Drop RpcProxy usage from metering_agent"