Describe the solution you'd like
Now SumDBs will proxy sumdb direct to the accually sumdb server insteads of behine the value of GlobalEndpoint

SumDBs = ["https://sum.golang.org"]  => the sumdb id is sum.golang.org
SumDBs = ["https://goproxy.cn/sumdb/sum.golang.org"]  => the sumdb id is goproxy.cn

Describe alternatives you've considered

What 's the best way to describe?

1. if the SumDB match https://(?<proxyHost>.*)/sumdb/(?<sumdb id>.*), use id sumdb id
2. all sumdb will try if upstream server support, then proxy to upstream/sumdb/id