[log parts enhance] Use env to control ilogtail log level

[zhu733756]

fixes #959

dev steps:

• follow the issue, and make this pr.

how to check it

• use devcontainer to develop, follow the article here
• export LOGTAIL_LOG_LEVEL=debug
• ./ilogtail &
• see the log prints like below:

read log level from the env successfully, level: debug
Load log config from /root/go/src/github.com/ilogtail/core/build/apsara_log_conf.json
Logger size in config: 4
Path of logger named /: /workspaces/ilogtail/core/build/ilogtail.LOG
logger named / created.
Path of logger named /apsara/sls/ilogtail: /workspaces/ilogtail/core/build/ilogtail.LOG
logger named /apsara/sls/ilogtail created.
Path of logger named /apsara/sls/ilogtail/profile: /workspaces/ilogtail/core/build/snapshot/ilogtail_profile.LOG
logger named /apsara/sls/ilogtail/profile created.
Path of logger named /apsara/sls/ilogtail/status: /workspaces/ilogtail/core/build/snapshot/ilogtail_status.LOG
logger named /apsara/sls/ilogtail/status created.

dalaos look here

Hi dalaos, I am a new comer for cpp developer, if this pr has any problem to run, please corret me, thanks in advance!

[CLAassistant]

All committers have signed the CLA.

[yyuuttaaoo]

There will be a dev meeting next week, would you like to join us and introduce your work?
see #1084

[zhu733756]

There will be a dev meeting next week, would you like to join us and introduce your work? see #1084

Thanks, It's my pleasure, I will attend this meeting if I have the time

[yyuuttaaoo]

Please add a change log line in CHANGLOG.md and help to add a doc to clarify the new env you've added here. https://github.com/alibaba/ilogtail/blob/main/docs/cn/configuration/system-config.md

[zhu733756]

Please add a change log line in CHANGLOG.md and help to add a doc to clarify the new env you've added here. https://github.com/alibaba/ilogtail/blob/main/docs/cn/configuration/system-config.md

done

[yyuuttaaoo]

Can you tell me how to contract you on DingTalk?
If you are using iLogtail in you company, may you record you use case here? #693

[zhu733756]

Can you tell me how to contract you on DingTalk? If you are using iLogtail in you company, may you record you use case here? #693

• My DingTalk name is 朱晗，I have joned the ilogtail社区 community
• I‘m sorry for that I am not responsible for observability development in the company. I am a cloud database development engineer. Personally, I am interested in both this project and observability projects. I would like to participate in open-source projects to enhance my skills. If possible, I will recommend it to them.