A common word used online is trending to indicate something is becoming noteworthy. The word trendy means something that is currently popular.
Of or in accord with the latest fad or fashion: trendy clothes.
The Free Dictionary by Farlex
You have since edited your question title as a way to dismiss trending as the word you are looking for. But I think you are misunderstanding how the word is used.
Technically, trending can mean either up or down. But, it's current usage by online sites is always taken to mean in the upward direction. When tracking trends, these sites use statistical measurements to decide if something is about to become more popular than it already is. If something already has a mass following in the millions, then an upward tick of 1000 additional hits would hardly be worth mentioning. But, if something only has a following in the tens of thousands, then an upward tick or 1000 is noteworthy, and the item would show up as trending.
Thus, the only known state for something that is about to become trending is that its popularity is stable and relatively unknown. At the moment the popularity moves upward from what it is, it becomes trending. When it reaches a critical mass of followers, it is considered popular.
For your song example, there is really no telling how a station decided a song was bubblare. On American radio, I would say this is analogous to "feature a song," or "put a spotlight on a song." This is more a marketing tool than a measurement of what is about to become popular. Producers try to influence stations to give their artists air time to promote their songs in the hopes it will make their artists popular (and them money).