这里的as,引导【非限制性的英语从句】,与后面内容链接,构成as固定搭配的表达形式.
类似固定搭配：
as is known to all
as is mentioned above
as is often the case
as is generally believed to be the case 正如普遍所认为得那样
as is expected to be the case
as的意思是【正如、、、、、、一样】.
例如：
As is known to all,China is still a developing country.
The elephant is like a snake,as anybody can see.
祝你开心如意!