Measure decibels while recording video or in playback

I have an iPhone SE.

Is there a way to measure the decibel as video is being recorded, and include it as part of the captured video? I want to show that there is something wrong with the mounting of a washroom ventilation fan. It is extremely loud after the motor was replaced.

The link https://www.reddit.com/r/ProAudiovisual/comments/5mb7kn/how_can_i_determine_actual_decibel_levels_of indicates that it is not possible, but it applies to playback. Therefore, it must be referring to audio coming from a recording, and apparently after automatic gain control (AGC). In contrast, what I'm seeking is a measurement right off the mic.

On a related matter, the above link also seems to indicate that AGC is applied as the audio is coming in, i.e., *before* the signal actually gets captured as a recording. Why would AGC be applied before capture rather than after? It seems that this compromises the fidelity of the recording. Applying AGC on the playback seems to achieve the same thing without compromising the fidelity of the capture.

P.S. If I can't do what I seek, I will fall back to the "RTA Audio" suggested in the above thread, or a CDC NIOSH app (https://www.noisyplanet.nidcd.nih.gov/have-you-heard/cdc-niosh-app).