Format gcloud compute instances list to get single metadata value - google-cloud-platform

Question

As part of some GCP admin automation I'm trying to run a gcloud compute instances list command to return a few instance properties, one of which is a single metadata property. I cannot find in the documentation how to return only a single metadata property.
This is what I would think is correct based on the doc, but I don't get any metadata properties returned...
gcloud compute instances list --filter="name~^my-machine.*-type" --zones=zone1,zone2,zone3 --format="json(name,metadata.items.MY_VALUE)"
How can I return a single metadata value?

Answer 1

Eesh... this was not obvious ;-)
KEY=...
gcloud compute instances list \
--project=${PROJECT} \
--format="value(metadata.items.extract("${KEY}"))"
See extract
I'm not sure why it works.
In my case:
gcloud compute instances list \
--project=${PROJECT} \
--format="value(metadata.items)"
Yields:
{'key': 'gce-container-declaration', 'value': "..."};
{'key': 'google-logging-enabled', 'value': 'true'}
NOTE a semi-colon separated list of JSON objects
So, metadata.items appears to be a list of JSON objects {"key": $KEY, "value": $VALUE} and I think this why you can't walk down through the values using something ... metadata.items.key["google-logging-enabled"] or similar.
When I initially looked using YAML, this wasn't obvious and I think, even though the YAML looks flat, the items are embedded and the --format=yaml is doing something clever:
gcloud compute instances list \
--project=${PROJECT} \
--format="yaml(metadata.items)"
---
metadata:
items:
- key: gce-container-declaration
value: |-
...
- key: google-logging-enabled
value: 'true'
But:
gcloud compute instances list \
--project=${PROJECT} \
--format="value(metadata.items.extract("gce-container-declaration"))"
Yields:
spec:
containers:
- name: instance-1
image: ...
stdin: false
tty: false
restartPolicy: Always