Enable runtime config log level

[Junchao-Mellanox]

Why I did it

SONiC provides two Python logger implementations: sonic_py_common.logger.Logger and sonic_py_common.syslogger.SysLogger. Both of them do not provide the ability to change log level at real time. Sometimes, in order to get more debug information, developer has to manually change the log level in code on a running switch and restart the Python daemon. This is not convenient.

SONiC also provides a C/C++ logger implementation in sonic-platform-common.common.logger.cpp. This C/C++ logger implementation is also a wrapper of Linux standard syslog which is widely used by swss/syncd. It provides the ability to set log level on fly by starting a thread to listen to CONFIG DB LOGGER table change. SONiC infrastructure also provides the Python wrapper for sonic-platform-common.common.logger.cpp which is swsscommon.Logger. However, this logger implementation also has some drawbacks:

1. swsscommon.Logger assumes redis DB is ready to connect. This is a valid assumption for swss/syncd. But it is not good for a Python logger implementation because some Python script may be called before redis server starting.
2. swsscommon.Logger wraps Linux syslog which only support single log identifier for a daemon.

So, swsscommon.Logger is not an option too.

This PR is a Python logger enhancement which allows user setting log level at run time.

Work item tracking

• Microsoft ADO (number only):

How I did it

swsscommon.Logger depends on a thread to listen to CONFIG DB LOGGER table change. It refreshes log level for each logger instances once the thread detects a DB entry change. A thread is considered heavy in a python script, especially that there are many short and simple python scripts which also use logger. To keep python logger light weight, it uses a different design than swsscommon.Logger:

• A class level logger registry shall be added to SysLoggerclass
• Each logger instance shall register itself to logger register if enables runtime configuration
• Logger configuration shall be refreshed by CLI which send a SIGHUP signal to the daemon

How to verify it

Manual test
New unit test cases

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[prgeor]

@Junchao-Mellanox can we agree on the HLD first before implementation?

[prgeor]

@Junchao-Mellanox Please use singleton class for SysLogger that way each process will have access to this singleton class and you can update the log level on receiving the signal.

[Junchao-Mellanox]

ci passed 4246