Farid Boulaya drilled a long-range free kick to give Metz an early lead, but his goal was overshadowed minutes later by a memorable strike from Khazri, measured at 68 yards according to Opta.
Khazri dribbled up the field after recovering the ball in his own penalty area and hammering it back onto the backpedal Alexandre Oukidja for his fourth goal in as many games, and his seventh this season.
“I’ve tried it a lot, sometimes they laugh at me, but when it comes out it’s good. It’s mostly good for the team,” Khazri told Prime Video.
“I moved the ball forward and didn’t have too many options, I tried and I got it.”
Saint-Etienne have drawn in three of their last four games but remain low in the standings and without a win in 12 games of the season. Metz is only a point above them in 18th place.
Lyon bounced back from a tough loss at Nice last weekend, beating second-placed Lens 2-1 to move up one point from the top three.
Karl Toko Ekambi scored a penalty and Houssem Aouar added a second for the hosts before half-time. Arnaud Kalimuendo brought back a goal for Lens, who lost for the second time in three games.
Lens leaders Paris Saint-Germain gained 10 points after Marquinhos and Angel Di Maria scored late goals in a 2-1 home win over defending champions Lille on Friday.
Nice, Marseille and Rennes could all pass over Lens with wins on Sunday.