On an early March day cloaked in gray skies, Selena Gomez, Steve Martin and Martin Short are taking a stroll down a bustling New York City block — it’s the start of filming for the fourth season of “Only Murders in the Building.” In the scene, they witness a series of vignettes: A guy who looks like a background actor in “A Bronx Tale” hollering at a taxi that won’t stop; an annoyed kid in a Knicks jersey evading his mother’s calls from the fire escape of an apartment overhead, where a sign hangs that says “Diversity”; and a young couple who failed to gaze up at the imaginary Godzilla in the d…