## Issue Addressed
Closes#1996
## Proposed Changes
Run a second `Logger` via `sloggers` which logs to a file in the background with:
- separate `debug-level` for background and terminal logging
- the ability to limit log size
- rotation through a customizable number of log files
- an option to compress old log files (`.gz` format)
Add the following new CLI flags:
- `--logfile-debug-level`: The debug level of the log files
- `--logfile-max-size`: The maximum size of each log file
- `--logfile-max-number`: The number of old log files to store
- `--logfile-compress`: Whether to compress old log files
By default background logging uses the `debug` log level and saves logfiles to:
- Beacon Node: `$HOME/.lighthouse/$network/beacon/logs/beacon.log`
- Validator Client: `$HOME/.lighthouse/$network/validators/logs/validator.log`
Or, when using the `--datadir` flag:
`$datadir/beacon/logs/beacon.log` and `$datadir/validators/logs/validator.log`
Once rotated, old logs are stored like so: `beacon.log.1`, `beacon.log.2` etc.
> Note: `beacon.log.1` is always newer than `beacon.log.2`.
## Additional Info
Currently the default value of `--logfile-max-size` is 200 (MB) and `--logfile-max-number` is 5.
This means that the maximum storage space that the logs will take up by default is 1.2GB.
(200MB x 5 from old log files + <200MB the current logfile being written to)
Happy to adjust these default values to whatever people think is appropriate.
It's also worth noting that when logging to a file, we lose our custom `slog` formatting. This means the logfile logs look like this:
```
Oct 27 16:02:50.305 INFO Lighthouse started, version: Lighthouse/v2.0.1-8edd9d4+, module: lighthouse:413
Oct 27 16:02:50.305 INFO Configured for network, name: prater, module: lighthouse:414
```
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