Atlas will be performing traffic recordings as part of shadow clusters.

Currently, enabling this requires setting trafficRecordingDirectory with a valid directory at startup; it cannot be changed at runtime.

This is with good reason - it ensures users with only privileges to start a recording can't write to arbitrary directories on the host with the permissions of the user running mongos/mongod.

However, this is impractical for Atlas, as a change would require a rolling restart of all nodes.

As a middle ground, allow it to be changed at runtime.
This avoids startTrafficRecording alone being able to write to arbitrary dirs, instead requiring the user also be sufficiently privileged for setParameter.
Many other "hazardous" parameters can be changed by users with such privileges.