improve request logging (closer to Apache Common Log)

contrib/google_auth_proxy.cfg.example

@@ -13,6 +13,9 @@
 #     "http://127.0.0.1:8080/"
 # ]
 
+## Log requests to stdout
+# request_logging = true
+
 ## pass HTTP Basic Auth, X-Forwarded-User and X-Forwarded-Email information to upstream
 # pass_basic_auth = true
 ## pass the request Host Header to upstream
@@ -51,3 +54,4 @@
 # cookie_expire = "168h"
 # cookie_secure = true
 # cookie_httponly = true
+