https://github.com/openstack/kolla-ansible/blob/428acfe97fdfbc89fd4dea577160192647e6f6b5/ansible/roles/prometheus/defaults/main.yml#L226

You'd have to completely redefine the default volumes list.  The other option is to do your mount into the /var/lib/docker/volumes/prometheus_v2/data directory
From: Satish Patel <satish.txt@gmail.com>
Sent: 15 September 2023 03:25
To: OpenStack Discuss <openstack-discuss@lists.openstack.org>
Subject: [kolla-ansible]change prometheus data volume path
 

CAUTION: This email originates from outside THG

Folks,

I have deployed prometheus with koll-ansible and now I want to change the path of prometheus volume to store matrices on bigger partitions. How do I change the volume path of prometheus to store matrix data on separate filesystem? 

This is what I did in globals.yml but it doesn't work. Is the following variable correct? 

prometheus_server_default_volumes: "/mnt/prometheus"