AWS ES Custom Endpoint does not work with AWS SignV4

[CompuGlobalHyperMegaNet-VP]

Hi,

recently, AWS introduced Custom Endpoints:
https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/es-customendpoint.html
so one can assign a permanent custom DNS record to the ES domain.

When using aws-es-proxy with the custom endpoints and SignV4 enabled, this fails:
time="2020-27-11 09:25:35" level=error msg="Received 403 from AWSAuth, invalidating credentials for retrial" 2020/11/27 09:25:35 -> GET; 172.12.13.14:42722; /_cluster/health; ; 403; 0.103s
whereas the same environment works fine on the normal endpoint.

Seems that proxy tries to extract region and partition from the given endpoint:

aws-es-proxy/aws-es-proxy.go

Line 179 in d26a388

// Extract region and service from link. This should be save now

which will fail for a custom endpoint.

BR