Permanent redirect to from instance to instance/

In a firebase function, which is just a wrapper around a Google Cloud Function, whenever the firestore code is fetching the project details, which triggers an `isAvailable` call. I see that there's a permanent redirect (301) from either:
`http://169.254.169.254/computeMetadata/v1/instance -> http://169.254.169.254/computeMetadata/v1/instance/`
`http://metadata.google.internal./computeMetadata/v1/instance -> http://metadata.google.internal./computeMetadata/v1/instance/`

These calls are rather slow and have high variance, taking between 20 and 200ms (I have seen times larger than a couple of seconds too).

I think that the code in the `metadataAccessor` (https://github.com/googleapis/gcp-metadata/blob/3f1eadacaa36f04aa1f105050031f6c158330576/src/index.ts#L67), should be changed so that when there's no property the url ends with a trailing slash, at least for the `instance` type.

See the traces below

### Environment details

- OS: Firebase Functions
- Node.js version: 10.18.1
- npm version: 6.14.4
- `gcp-metadata` version: 3.5.0

#### Steps to reproduce

1. Make a firestore request in a Firebase Function
2. Check the traces of the http request