The MySQL slow query log is a difficult format to extract information from. After looking at various examples with mixed results, I realized that it's much easier to configure MySQL to write the slow query log to a table in CSV format!
From the MySQL documentation:
By default, the log tables use the CSV storage engine that writes data in comma-separated values format. For users who have access to the .CSV files that contain log table data, the files are easy to import into other programs such as spreadsheets that can process CSV input.
Note: don't forget to open up permissions on your slow query log CSV file so logstash can read it!
# enable slow query log