Name
WPSOLR_Events::WPSOLR_FILTER_POST_CUSTOM_FIELDS
Short description
Modify custom fields before they are indexed.
Long description
A Solarium update document containing the post custom fields is ready to be sent to the Apache Solr index.
You want to change some of the custom fields (add/update/remove).

Examples
Search in the plugin's code (we eat our own dog food).