This is likely too strong
I'll be using Detect Balance to score this, to get as quantitative as possible. Detect Balance is not panacea as it fails to capture synergies between different abilitites, but I do not see any here, so it should work fine. And it can do a better job of assessing the cumulative power of several "harmless" abilities that make a race better, than just looking for broken stuff does.
Everything except for Life Leech is standard, and the scores sum up to 14 points:
Sinful: I cover this under Humanoid + type (that is, the race is humanoid, which is the standard and makes it vulnerable to all the spells that only can target humanoids, but in addition works as another type for some circumstances). This is worth -1 point.
Languages: Demons in 5e typcally speak Abyssal. Devils speak Infernal. Yugoloths (formerly known as Daemons) and Succubi speak both Abyssal and Infernal. There is no language called "Daemonic". So I scored this as if your intention was that the race can speak both Abyssal and Infernal, like Succubi or Daemons, as "Common & 2 or 3 languages", worth 1 point.
False Friends. The cantrip is worth 2 points.
Attribute increases These are worth 8+4 or 12 points. All the rest of the common features are standard and worth 0 points.
The ideal number recommended for a balanced homebrew race is 25 points, with a recommended range of 24-27. That allows for 10 to 13 points for Life Leech.
Advantage. First, the way it is written, the Cambion can target themselves and trade a few hit points for adavantage on their next roll whenever they want to. That flexibility in itself is powerful, because you can choose what is more important in a given situation. It is gated by a con save that cannot be voluntarily failed.
To score the value of life leech, I'll compare it to the Lucky feat, which grants you something quite close to advantage on three rolls of any kind per day. Life leech allows you to get advantage on up to six rolls per day depending on your level, but works not always due to the save. Lucky is worth 8 points, and the ability to give advantage at will may be worth up to 8 points, like "Advantage on a common roll or set of rolls". (I also can easily imagine the Cambion having low con "blood pets", like small animals in a cage, to leech life whenever needed, offsetting the disadvantages to use it in non-combat situations and granting high probability save failure).
Damage. The ability can deal 4 to 12 points of necrotic damage as a bonus action at 30 feet range, and that is pretty unprecedented.
As a benchmark, look at the dragonborn's breath weapon attack with a line weapon: also 30' range, a comparable save, and a comparable number of uses - it resets on short rest, so Life leech is slightly better). Breath weapon deals a little more damage of a weaker damage type. (It also can hit multiple targets if you are lucky, but is limited by not working well if your allies are in line, so that is pretty situational). In general, I'd say it is about as strong up to here. However, a breath weapon costs your action, which makes Life Leech a lot better, as you can do it in additon to your normal action. Breath Weapon is already worth 6 points, so the damage aspect here is worth at least 6 points.
I am not aware of any spell or cantrip that allows you to do directly deal damage at range as a bonus action, they all need an action. The closest may be Spiritual Weapon at second level, which requires an attack roll instead of a save. Spells like hex or hunter's mark at first level do similar damage from a bonus action, but the target needs to be hit with separate attacks for it. If this was a first-level spell useable once per day, it would cost 3 points. Considering you can do it 2-6 times, this is again worth at least 6 points, and more likely 9 points or more (most play happens in tier one, two and early levels of tier three, for an average of 3 uses).
Adding up the minimal 8 + 6 points, we get 14 points for Life Leech and with the other 14 points a total of 28, putting you over the recommended range. A probably more adequate value of 8 + 9 or 17 points puts your race at 31 points, far above recommended range.
You also need to consider the synergy of getting both advantage and dealing damage from the same bonus action, which further helps your action economy. The point values above are without adding anything for this synergy, which is also pointed out as powerful in J.A. Streich's answer.
I think it is too strong, and if you are open for recommendations, you could turn the ability into an action instead of a bonus action — that seems more natural looking at breath weapons, and a better fit for the amount of power it packs.