CST_ Screen Media presents a documentary directed by Marilyn Agrelo. Rated PG (for some thematic elements, language and smoking). Running time: 107 minutes. Opens Friday at Landmark Century Centre and available May 7 on demand. That’s the thing about “Sesame Street.” It’s in our heads and in our hearts and in our memories forever, and we’re all the better for it. So yes, the documentary “Street Gang: How We Got to Sesame Street” is an unabashed love letter to the first 20 years of the most influential, most beloved and most enduring children’s program in television history, and why not? Nobody wants to see an expose ripping the lid off Oscar the Grouch’s trash can to tell the real story about the longstanding feud between Bert and Ernie and the ugly truths about Big Bird’s prima donna behavior, and I’m just kidding, none of that ever happened! By all accounts, Big Bird was a real sweetheart to work with, and Bert and Ernie got along just fine off set.