Here's the real answer:
You need to use add an OSLogPreferences dictionary to your app's "Info.plist" file.
For details, open Terminal and type man 5 os_log. The general structure will be something like this:
<key>OSLogPreferences</key>
<dict>
<key>put.your.subsystem.name.here</key>
<dict>
<key>put.your.category.name.here</key>
<dict>
<key>Level</key>
<dict>
<key>Enable</key>
<string>Debug</string>
<key>Persist</key>
<string>Debug</string>
</dict>
<key>Enable-Private-Data</key>
<true/>
</dict>
</dict>
</dict>
In Xcode, this looks like this:
You enter a subsystem and category when you instantiate your Logger in Swift, something like this:
import OSLog
let myLogger = Logger(subsystem: "com.example.mycompany.mysubsystem", category: "kittens")
logger.log("Hello there!")
The dictionary under OSLogPreferences can have multiple subsystems, and each subsystem can list one or more categories. You can also use the special category key, DEFAULT-OPTIONS, to define common settings for all categories in a subsystem. So, if you used the single subsystem, com.example.mycompany.mysubsystem, you could do something like this:
<key>OSLogPreferences</key>
<dict>
<key>com.example.mycompany.mysubsystem</key>
<dict>
<key>DEFAULT-OPTIONS</key>
<dict>
<key>Level</key>
<dict>
<key>Enable</key>
<string>Debug</string>
<key>Persist</key>
<string>Debug</string>
</dict>
<key>Enable-Private-Data</key>
<true/>
</dict>
</dict>
</dict>
In both of my examples, the Enable-Private-Data key isn't required, but I include it so that everything doesn't say when you try to look at your logs. Thanks to @eskimo for pointing me to this man page.
