Similarly to mtail, grok_exporter parses unstructured log data and generates metrics from it. However, as the name suggests, the main difference is the domain-specific language for this exporter being modeled after the Logstash pattern language (Grok), which enables the reuse of patterns you might already have built.
The source code and installation files for grok_exporter are available at https://github.com/fstab/grok_exporter.