Design critics have been raving about Chicago’s new Millennium Park. But tonight is the real tes, when the first concert is given in the Frank Gehry-designed structure. “The pavilion’s most distinctive features are the curled, stainless steel panels that frame the stage like a bank of gray, shimmering clouds. The stage itself, ample enough for a 100-member orchestra and 150-member chorus, is lined with light reddish-brown Douglas fir. Fixed seating for 4,000 stretches back from the stage, and Millennium Park officials estimate that the lawn, crisscrossed by a metal trellis that holds the amplification system, can hold up to 7,000 more listeners.”