How to DescribeVPCs in a particular AWS region using Go AWS SDK?

I want to query all VPCs belonging to a particular region in my Go-based microservice.

https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeVpcs.html indicates that there exists no filter option by region or any other request parameter.

Golang SDK reference document:
https://docs.aws.amazon.com/sdk-for-go/api/service/ec2/#EC2.DescribeVpcs

Command line SDK reference document: https://docs.aws.amazon.com/cli/latest/reference/ec2/describe-vpcs.html

Here's an example.

{
    "Vpcs": [
        {
            "CidrBlock": "30.1.0.0/16",
            "DhcpOptionsId": "dopt-19edf471",
            "State": "available",
            "VpcId": "vpc-0e9801d129EXAMPLE",
            "OwnerId": "111122223333",
            "InstanceTenancy": "default",
            "CidrBlockAssociationSet": [
                {
                    "AssociationId": "vpc-cidr-assoc-062c64cfafEXAMPLE",
                    "CidrBlock": "30.1.0.0/16",
                    "CidrBlockState": {
                        "State": "associated"
                    }
                }
            ],
            "IsDefault": false,
            "Tags": [
                {
                    "Key": "Name",
                    "Value": "Not Shared"
                }
            ]
        }
    ]
}

However, if I use the command

$aws ec2 describe-vpcs --region us-west-1

then I can query all vpcs in region us-west-1.

Question 1. Why is the --region option not mentioned in the CLI SDK document?

Question 2. How can I incorporate the same in DescribeVpcsInput while using GO SDK?

The --region flag on the CLI is not a filter, it is a required setting that tells the AWS CLI what region to connect to. The ec2 describe-vpcs command is always limited to a single region (most AWS commands are).

You would configure your AWS SDK client with the region you want it to connect to as well. See "Specifying the AWS Region" here.