I was curious to see if there are any errors relating to temporary accounts after the recent Wikimedia-production-error T403631: TypeError: undefined is not an object (evaluating 'username.startsWith') but was unable to debug whether they came from temporary accounts or not. I assume it would be important to have this to monitor errors during roll out?

Steps to replicate the issue (include links if applicable):

• Go to https://logstash.wikimedia.org/goto/3cd0a7979a3759dc3d1952d90473e60b
• Expand an error

What happens?:
There is a error_context.is_logged_in field but no error_context.is_temp_account field, so I can't tell if the error was from a temporary user or not.

What should have happened instead?:

Fields should support filtering for temporary or logged in users

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

Code: https://gerrit.wikimedia.org/g/mediawiki/extensions/WikimediaEvents/+/315af3c97b6b1822d6aaa462c36b48222da353d6/modules/ext.wikimediaEvents/clientError.js#417