The Metrons are a highly advanced, long-lived civilization.
The Metrons were one of the early creations of the Preservers and initially warred with their fellow creations, the Organians, before the two species stood down and eventually became firm allies. (TOS - Year Four comic: "The Enterprise Experiment, Part 5")
The Federation's first contact with the Metrons was made in 2267 by Captain James T. Kirk of the USS Enterprise. Kirk had ordered his ship to pursue a Gorn vessel that had destroyed the colony and outpost on Cestus III. The Metrons forced Kirk and the Gorn captain to fight to the death to decide the outcome of the battle. When Kirk won the fight, but chose to spare his opponent's life, the Metrons returned both captains to their ships and transported both vessels away from their space. (TOS episode: "Arena")
Following this encounter, Spock erroneously theorized that the Metrons themselves were the Preservers before Ayelborne revealed the truth to him. (TOS - Year Four comic: "The Enterprise Experiment, Part 5")
By 2373, the Metron species was dying out. They lured Captain Benjamin Sisko of the USS Defiant to their space with a false distress call so he could help preserve their knowledge for future generations. (DS9 - Strange New Worlds I short story: "Where I Fell Before My Enemy")
Two years later, in 2375, then-Governor Nanietta Bacco of Cestus III negotiated a historic peace treaty between the Federation and the Metrons following an attack on the world by the Gorn Hegemony. (TNG novel: A Time for War, A Time for Peace)
In 2379, the Metrons, along with the Organians, were the victims of a preemptive strike by the Q for supremacy of the universe. Though the Q were objectively more powerful, the Metrons gave a good account of themselves, being able to stem the tide of their foes' advance with their conflict, projected to last for centuries, manifesting itself as supernovae in 3-D space. When Jean-Luc Picard protested this to Q, the entity abducted his and three other Starfleet crews to act as proxy armies. The Metron representative, despite his distaste for the whole charade, selected Kathryn Janeway to represent his people, her crew scoring the first victory in the contest by successfully retrieving an Iconian Gateway Engine. Via the interventions of Amanda Rogers, Wesley Crusher, and the Traveler however, the conflict was brought to a quick end, the Metrons agreeing to keep away from the Q as long as they extended the same courtesy. (TNG - The Q Conflict comics: "Issue 1", "Issue 2", "Issue 6")