Command `describe` does not include `metadata`.

**What steps did you take:**
As per https://github.com/carvel-dev/imgpkg/issues/124#issuecomment-823325183, I expect when we do `imgpkg describe -b ...`, if the bundle have `metadata` fields, it it will be printed as written in `imgpkg/bundle.yml`.

I put some data:
```
$ cat .imgpkg/bundle.yml
metadata:
oslManifestURL: ...
```
before I do `imgpkg push -b ...`, but it's not appearing when I do `imgpkg describe -b ...`:
```
$ imgpkg describe -b ... --output-type yaml
sha: sha256:bb59ca320f34063174bc9c00bc6f5b795d223c3b9b965851038ce85ebe4eb1d2
content:
...
image: ...
metadata: {}
origin: ...

Succeeded
```

**What happened:**
The `metadata` information is not printed.

**What did you expect:**
The `metadata` information is printed.

**Anything else you would like to add:**
The [code](https://github.com/carvel-dev/imgpkg/blob/870f8123934c43005e118c873ec447590309ab2a/pkg/imgpkg/v1/describe.go#L134) seems to never populate this, just put an empty map as place holder.

Can this please be rectified, so I can retrieve this information vie `impkg describe -b ...`?

If I do `imgpkg pull -b ...`, the retrieved `.imgpkg/bundle.yml` has the information, but I don't want to have to pull it as the bundle can be huge, it's just a waste of time and local storage to pull the whole bundle just for me to get the `.imgpkg/bundle.yml`.

**Environment:**

- imgpkg version (use `imgpkg --version`): 0.34.1
- Docker registry used (e.g. `Docker HUB`): Harbor, Artifactory
- OS (e.g. from `/etc/os-release`): Various (Ubuntu, CentOS, photon).

---
Vote on this request

This is an invitation to the community to vote on issues, to help us prioritize our backlog. Use the "smiley face" up to the right of this comment to vote.

👍 "I would like to see this addressed as soon as possible"
👎 "There are other more important things to focus on right now"

We are also happy to receive and review Pull Requests if you want to help working on this issue.