fix: correct datadir disk usage reporting in lighthouse.rs

[crStiv]

Fix datadir disk usage reporting in lighthouse.rs

Description

This pull request updates lighthouse.rs to fix the disk usage metrics. Previously, the disk usage was calculated for the root directory (/). The new implementation correctly retrieves and uses the LIGHTHOUSE_DATADIR environment variable to calculate disk usage for the appropriate data directory. If the environment variable is not set, it defaults to /.

Fixes #6687

Proposed Changes

• Modify the disk_usage calculation to use the LIGHTHOUSE_DATADIR environment variable.
• Add a fallback to the root directory if LIGHTHOUSE_DATADIR is not specified.
• Update the error handling to provide clearer error messages if disk usage information cannot be retrieved.

Additional Info

• This change introduces a behavior that depends on the LIGHTHOUSE_DATADIR environment variable. Reviewers should ensure the variable is correctly set in environments where this change is deployed.
• Future consideration: Test cases could be added to validate behavior when LIGHTHOUSE_DATADIR is missing or set to invalid paths.

Allow edits by maintainers

Yes.

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[michaelsproul]

This should not require the addition of a new environment variable. Lighthouse already knows the data directory from the --datadir CLI flag.

[crStiv]

@michaelsproul this one seems to be better