PHPC-2180 defines an internal MongoDB\Driver\Monitoring\mongoc_log function for internal testing. Considering only conditionally defining this function when the build is configured for testing (e.g. check a defined C constant).

While this is relatively straightforward to do with config.m4 (e.g. alongside building with developer flags), it would require more effort for Windows builds since we currently use the same build for both testing and releases.

Note: this came out of mongodb/mongo-php-driver#1395.