The classic phrase for such a description is play-by-play. Literally this means a detailed description of a sports event, describing each event in the game as it happens. But it has been extended to descriptions of other sorts of events, including political ones, on a similar as-it-happens basis.
Such "play-by-play" descriptions arose when games were broadcast by radio, with no accompanying images, because video had not yet been developed. The intent was to give the listeners the same information as they would have if they were physically present. Indeed it gave more, because the broadcaster often had expert knowledge of what to watch for, what was significant, that many people did not have, or not to the same degree.
It is also known as "play-by-play commentary." Strictly speaking the commentary is the account of the event, not the process of delivering that account, but it is often extended to cover the process as well.
The action of doing the description is sometimes called "giving a play-by-play" or "delivering a play-by-play". In either case, "commentary" can be added.