The metadata doesn't need to be exclusive to a single target - multiple targets can be assigned the same metadata. And yes, you can custom define your metadata. The intention is for the metadata to enable you to associate a search result with both a target and custom defined data about that target.

You should use the new keys, we have migrated your project to our Production platform. And it will be a good idea to use the newest SDK release. There are fixes and other improvements in the current release. 

I am using unity vuforia 2-0-18, so you mean metadata package is something similar to unique id of one image? and i can input any number i want as long as it refer to base64 encoding?

another question, i have an access and secret key you gave me with unity vuforia 2-0-18, but today i build a cloud database via vuforia target manager ( i am surprised with new feature here ), but the access and secret key i got here is different..which one should i use???

Whatever metadata you associate with your target will be returned with its search result. The metadata package is restricted to 150KB or 200KB if provided as a base64 encoding. See: https://developer.vuforia.com/resources/dev-guide/managing-targets-cloud-database-using-developer-api

Which SDK are you using?