Clarification required to make use of --labels

Right now i don't get the documentation about how to use the option flag --labels.
It states:
```
Additional flags. Can be accessed in feedback and mirror context objects via the cli_labels field. In core.workflow, they are accessible as labels, but with names uppercased and prefixed with FLAG_ to avoid name clashes with existing labels. I.e. --labels=label1:value1 will define a label FLAG_LABEL1Format: --labels=flag1:value1,flag2:value2 Or: --labels flag1:value1,flag2:value2
```

But if i try to access labels within the workflow i only get:
`name 'labels' is not defined`.

I tried using it within the workflow and outside, but now luck. Probably i get something wrong.

An example would look like:
```
url = "https://github.com/google/copybara.git"

core.workflow(
name = "default",
origin = git.origin(
url = url,
ref = "master",
),
destination = git.destination(
url = "file:///tmp/foo",
fetch = labels.get("FLAG_DESTINATION_FETCH", "main"),
push = "master",
),
# Copy everything but don't remove a README_INTERNAL.txt file if it exists.
destination_files = glob(["third_party/copybara/**"], exclude = ["README_INTERNAL.txt"]),

authoring = authoring.pass_thru("Default email "),
transformations = [
core.move("", "third_party/copybara"),
],
)
```

Which i invoked like:
`java -jar copybara.jar copy.bara.sky --labels destination_fetch:develop` in hopes to be able to dynamically change the actual fetch source. Same idea would apply for push and ref.