Allow for easier modification of log4rs-based logging

[christophermaier]

As a follow-on to #6583, if we decide to continue to allow logging modification through environment variables, we should also deprecate RUST_LOG in favor of HAB_LOG (the fact that Habitat is written in Rust shouldn't matter to users, and the HAB_ prefix is consistent with our other environment variables).

We will need to iron out exactly how this would interact with a configuration file. Would it take priority over the file, or would the file take priority over it (particularly since the latter can be modified at runtime, but the former cannot)? Would the environment variable configuration be merged with anything in a file? Should the environment variable continue to contain a limited subset of logging configuration (consistent with its previous env_logger use), or would we want to do something like accept the contents of the variable as a valid log4rs YAML file?

[christophermaier]

Alternatively, we could look at this as providing some basic log configuration functionality in Habitat's own terms (independent of any underlying logging system implementation). "Power users" that are willing to dig into implementation-specific details would still be free to manipulate their own log4rs configuration file, recognizing that that is not necessarily an interface contract that we will always provide.

[stale]

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. We value your input and contribution. Please leave a comment if this issue still affects you.