fix: release buffer when send log messages after max retry times

[nic-6443]

When using UDP as the transport protocol, due to the payload size limitation of UDP (65507), if there is a log line that exceeds this size limit and attempts to send, _do_flush call fails withEMSGSIZE. However, since send_buffer is not reset, all subsequent logs will still include the previous oversized log when sent again, causing continuous errors.
refs:

lua-resty-logger-socket/lib/resty/logger/socket.lua

Line 155 in 5820faf

self.send_buffer = self.send_buffer .. packet

lua-resty-logger-socket/lib/resty/logger/socket.lua

Line 257 in 5820faf

bytes, err = _do_flush(self)

lua-resty-logger-socket/lib/resty/logger/socket.lua

Line 289 in 5820faf

self.send_buffer = ""

[membphis]

according this:

I think we need to control the max payload size, change some code about this

but we only change this line:

pls confirm you submit all of the relation code, it confused me

[nic-6443]

@membphis @shreemaan-abhishek

I think we need to control the max payload size

This lib already has such a feature, flush_limit is used to control the buffer to write logs into the socket when it reaches a certain size:

lua-resty-logger-socket/lib/resty/logger/socket.lua

Lines 341 to 342 in 5820faf

	flush_limit = 4096, -- 4KB
	drop_limit = 1048576, -- 1MB

. But the current problem is that in abnormal scenarios, a single log exceeds the limit of the UDP protocol.

By the way, we also cannot split the data here and send it multiple times because the log data itself has an encoding protocol.
https://github.com/apache/apisix/blob/bcf0c044d573ac9a0bfed07746728b31253b46e5/apisix/plugins/syslog/init.lua#L89-L90
If it is sent dispersed across multiple UDP sockets, it will become multiple logs on the syslog server side.

The main cause of this issue is that the user enabled the collection of request body and response body while using APISIX's syslog plugin, but did not set a size limit (we have informed the user to configure this limit). This caused the syslog plugin to attempt to use this library to send data exceeding the UDP protocol limit. Due to the bug mentioned in the PR description, it further led to subsequent logs that conform to protocol size also being unable to be sent.