startuptime is now reported when it isn't exceeded.

limitenforcer's enterStartup* functions now receive a reference to a
OneOf an exception or a duration. Which can be populated by the elapsed
duration if the limit was not exceeded.

src/workerd/io/limit-enforcer.h

@@ -32,24 +32,24 @@ class IsolateLimitEnforcer {
   // When the returned scope object is dropped, if a limit was exceeded, then `error` will be
   // filled in to indicate what happened, otherwise it is left null.
   virtual kj::Own<void> enterStartupJs(
-      jsg::Lock& lock, kj::Maybe<kj::Exception>& error) const = 0;
+      jsg::Lock& lock, kj::OneOf<kj::Exception, kj::Duration>& limitErrorOrTime) const = 0;
 
   // used to enforce limits on Python script startup.
   virtual kj::Own<void> enterStartupPython(
-      jsg::Lock& lock, kj::Maybe<kj::Exception>& error) const = 0;
+      jsg::Lock& lock, kj::OneOf<kj::Exception, kj::Duration>& limitErrorOrTime) const = 0;
 
   // Like enterStartupJs(), but used when compiling a dynamically-imported module.
   virtual kj::Own<void> enterDynamicImportJs(
-      jsg::Lock& lock, kj::Maybe<kj::Exception>& error) const = 0;
+      jsg::Lock& lock, kj::OneOf<kj::Exception, kj::Duration>& limitErrorOrTime) const = 0;
 
   // Like enterStartupJs(), but used to enforce tight limits in cases where we just intend
   // to log an error to the inspector or the like.
   virtual kj::Own<void> enterLoggingJs(
-      jsg::Lock& lock, kj::Maybe<kj::Exception>& error) const = 0;
+      jsg::Lock& lock, kj::OneOf<kj::Exception, kj::Duration>& limitErrorOrTime) const = 0;
 
   // Like enterStartupJs(), but used when receiving commands via the inspector protocol.
   virtual kj::Own<void> enterInspectorJs(
-      jsg::Lock& lock, kj::Maybe<kj::Exception>& error) const = 0;
+      jsg::Lock& lock, kj::OneOf<kj::Exception, kj::Duration>& limitErrorOrTime) const = 0;
 
   // Notifies the enforcer that a request has been completed. The enforcer is more lenient about
   // limits if several requests have been completed, vs. if limits are broken right off the bat.