Add a Typescript declaration file for metricsgraphics

[cnwangjie]

There are many libraries with a Typescript declaration file (a .d.ts file), it provides a definition of the type of api like c header file. If there is a declaration file you can get hints from the editor if you have a declaration file whatever writing Javascript or Typescript. And the declaration file will have comments. it hide the details of the code and leaving the api that can be called, which is very friendly to the user.

It is an example

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/koa/index.d.ts

and user can get a tooltip like this if users are using a modern editor like vscode or atom or using IDE.

Also, we can easily generate a document from it.