Mark Chavez, 54 has been charged in the investigation of the death of Friends star Matthew Perry and has today pleaded guilty to conspiring to distribute the surgical anesthetic ketamine. Chavez reportedly signed a plea agreement with prosecutors back in August and is the third person to plead guilty in the aftermath of the actor’s death. Chavez, entered the guilty plea on Wednesday in a federal court in Los Angeles, with prosecutors offering lesser charges to Chavez and two others in exchange for their cooperation. The court is set to go after two targets they deem more responsible for the ov…