The engagement determination will be based on whether the targetRateLimit (as seen in FTDC) is throttled to be less than the maximum. Every time the targetRateLimit transitions from "max" to "less than max", the "flow control engaged" message will be logged, and every time the targetRateLimit transitions from "less than max" to "max", the "flow control disengaged" message will be logged. Note that depending on the workload, these messages could be very noisy, so let's not introduce them at the default log level, but instead at debug log level 1.
This is a feature request. The mongod error log has messages to indicate when flow control is engaged. This request is to log when it is disengaged. That makes it easier to see when flow control might explain performance problems.
An example when it is engaged is: