WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 222, "The Night Before The Final Round," now streaming on Crunchyroll.
Given how big the cast of Konoha-nin was in the original Naruto, it was tough to juggle them all and allot equal screen time. Filler episodes helped with this but often, characters like Rock Lee quickly fell by the wayside as the Kaguya war, Akatsuki, Madara and Obito Uchiha all came into focus. Boruto is unfortunately suffering the same problem and as the anime unfolds, Metal Lee's biggest mystery clearly deserves its own arc in the wake of his biggest failure.
It occurs when Metal Lee fails to qualify for the finals of the Chūnin Exam. He's long wanted to graduate but he's stuck being a genin for now, as he's not up to the mental or physical requirements. However, just as Rock Lee persevered in the face of adversity, Metal Lee is upbeat and looking toward a brighter tomorrow.
His teammates on Team 5, Denki and Iwabee, feel bad and wonder if he's sulking, but Metal Lee shows up, ready to help them train. He doesn't care if he's out -- he wants the others to succeed so he can live vicariously through them. It's the kind of loyalty Naruto has inculcated in the Hidden Leaf in his years as Hokage, but it's bittersweet seeing Metal Lee so selfless and believing in the idea of family when Boruto hasn't addressed his history.
We know Rock Lee is his father, ergo his Taijutsu addiction, bowl haircut and costume, as well as his resilient attitude. After all, Metal Lee's dad used all this while he trained in Team Guy with Neji and Tenten, but the problem is that while Rock Lee is shown giving advice and such, Metal Lee's mother has been left out the picture.
The Boruto manga has completely ignored it too, which is shocking as all the shinobi of old paired off and had children. Many fans speculate it's Tenten, but she has never interacted with Metal Lee, nor has she shown any romantic connection to her ex-teammate.
Tenten went solo even during Naruto and Hinata's wedding, and ever since then, all couples have been seen while she's just been patrolling to help Naruto, Sasuke and Shikamaru's council protect their lands. Had she been Metal Lee's mother, we'd have seen her with Rock and Metal, plus Metal would have shown some of her wind and scroll skills in battle.
It's a total injustice as Tenten could have reminded the kid of the sacrifice Neji made by dying in the war against Obito's forces, which would connect Metal to Boruto as Neji was Hinata's cousin. Sadly, Metal Lee floats around without half of his identity in an annoying manner -- and Boruto fans may not care for the big reveal as it'll be too little, too late in terms of emotional investment.