add support for avg function (#50)

Author: gorried

be/src/query.rs

@@ -650,8 +650,9 @@ impl UserQuery {
 
     fn validate_function(&mut self, function: &mut ast::Function) -> Result<(), api::Error> {
         let name = function.name.to_string().to_lowercase();
-        const VALID_FUNCS: [&str; 15] = [
+        const VALID_FUNCS: [&str; 16] = [
             "coalesce",
+            "avg",
             "min",
             "max",
             "sum",