If we had a log ingestion rule on this log line, we could have a lot more insight into index build throughput and could answer questions like what is the 99th percentile for throughput of an index build, etc.

Currently, log ingestion rules are mostly used for crashes or other failures, and not for routine events like index builds; the current ingestion limit is 1000 requests per hour. Once we have insight into how often index builds are being completed (SERVER-135208) we can try to pursue creating an ingestion rule for this log.