Support proxy level access logging for udp proxy

[giantcroc]

Title: Support proxy level access logging for udp proxy

Description:

UDP proxy supports session access logging currently.
We can use log_format as followed to print stats in sessions.

[%START_TIME%] %DYNAMIC_METADATA(udp.proxy:cluster_name)%
%DYNAMIC_METADATA(udp.proxy:bytes_sent)%
%DYNAMIC_METADATA(udp.proxy:bytes_received)%
%DYNAMIC_METADATA(udp.proxy:errors_sent)%
%DYNAMIC_METADATA(udp.proxy:errors_received)%
%DYNAMIC_METADATA(udp.proxy:datagrams_sent)%
%DYNAMIC_METADATA(udp.proxy:datagrams_received)%\n

But some stats like no_route and idle_timeout can't be printed by session access logging,
because in some cases there is no session created.
So I think we should add proxy level log to enhance udp proxy access logging.
[optional Relevant Links:]

access log usage

[github-actions]

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