The coup plotters would have thrown up their hands in surrender. There were heavy-metal heavyweights AC/DC and Metallica and blues-rockers the Black Crowes playing a free concert on a Moscow airfield on Sept. 28. There were metalheads, many clad in universal metalwear: black T-shirts, leather jackets, crucifixes. The concert was, says sponsor Time Warner Inc., a ''gift to the youth of Russia'' for their support of democracy. The Russian group EST (Electroshock Therapy) joined in. Estimates of the crowd ranged from 150,000 to 850,000, including 12,000 metal-cautious security types. There were some skirmishes, mostly between cops and rowdy kids. But mostly there was music, mostly awesome. And there was philosophy, mostly deep. Like this from AC/DC's Brian Johnson: ''It takes rock & roll to make no more Cold War.'' What a country.