Allow to configure log level of Gitlab::Logger (by env variable)

What does this MR do and why?

There's an epic about "Setting Logging Verbosity" (&6034). Right now, you can't change the verbosity (level) of the Gitlab::Logger.

After a discussion in #331334 (comment 783581514), this MR makes the log level of Gitlab::Logger configurable using a new environment variable GITLAB_LOG_LEVEL.

• Before:
  • The log level for production is set to INFO in config/environments/production.rb#L40.
  • But when building a logger (based on Gitlab::Logger), the log level DEBUG is passed hard-coded in lib/gitlab/logger.rb#L36.
  • It turned out that only Gitlab::Geo::Logger and Gitlab::Geo::LogCursor::Daemon use the log level directly and all others use that from Gitlab::Logger.
• After:
  • The log level for production is set to DEBUG in config/environments/production.rb#L40.
  • When building a logger (based on Gitlab::Logger):
    • uses the log level in the new environment variable GITLAB_LOG_LEVEL if it exists
    • else: use the log level defined in the environment initializer for production in config/environments/production.rb#L40
  • So by default, the log level of Gitlab::Logger is still DEBUG but can now be configured using an env variable.
  • The log level for Gitlab::Geo::LogCursor::Daemon is set to INFO (hard-coded) because it should not be INFO by default (see #331334 (comment 803751056)).

(Please see #331334 (comment 783581514) for more information.)

The following values are valid for the GITLAB_LOG_LEVEL environment variable (according to the Ruby logger):

• 0, :debug, 'DEBUG' (case-insensitive)
• 1, :info, 'INFO' (case-insensitive)
• 2, :warn, 'WARN' (case-insensitive)
• 3, :error, 'ERROR' (case-insensitive)
• 4, :fatal, 'FATAL' (case-insensitive)
• 5, :unknown, 'UNKNOWN' (case-insensitive)

❓ Question: How and where should this be documented?

🛠 with ❤ at Siemens

/cc @bufferoverflow

How to set up and validate locally

1. Add an environment variable GITLAB_LOG_LEVEL = DEBUG and start GitLab instance.
2. Produce some logs of different levels ➡ DEBUG logs (and above) are emitted.
3. Change the environment variable GITLAB_LOG_LEVEL to INFO and restart GitLab instance.
4. Produce some logs of different levels ➡ only INFO logs (and above) but no DEBUG logs are emitted.

MR acceptance checklist

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