Without this fix, keys are prefixed with x-amzn-meta- by the TypeScript code,
then the Python code runs and prefixes metadata keys again with x-amz-meta-.
Then, the Python shells out to aws-cli which makes a request to the S3 service.
There, the keys are prefixed *yet again* with x-amz-meta- _unconditionally_.

Thus no matter what keys the user specified, the keys in S3 would be:
x-amz-meta-x-amz-meta-x-amzn-meta-<user input>.

This issue was originally reported as #8459.

After this change, neither the TypeScript code nor the Python code will attempt
to prepend to the metadata key. Instead we rely on the S3 service to do it.