plugins/rest/aws: debug log metadata error (#4061)

A user on Slack is getting 500 responses from the AWS metadata API
using a configuration like the below:

```yaml
 services:
  - name: svc-bundle
    url: https://opa-policies-dev.s3.amazonaws.com
    credentials:
      s3_signing:
        metadata_credentials:
          aws_region: us-east-1
          iam_role: arn:aws:iam::1234:role/opa-bundles
```
It's really hard to know why this might be given how we currently
only log the HTTP status code under this error condition.

This tries to print the response body on debug level (if set).

Signed-off-by: Anders Eknert <anders@eknert.com>

plugins/rest/aws.go

@@ -378,6 +378,12 @@ func doMetaDataRequestWithClient(req *http.Request, client *http.Client, desc st
 	}).Debug("Received response from " + desc + " service.")
 
 	if resp.StatusCode != 200 {
+		if logger.GetLevel() == logging.Debug {
+			body, err := ioutil.ReadAll(resp.Body)
+			if err != nil {
+				logger.Debug("Error response with response body: %v", body)
+			}
+		}
 		// could be 404 for role that's not available, but cover all the bases
 		return nil, errors.New(desc + " HTTP request returned unexpected status: " + resp.Status)
 	}