Adding Matomo to Trellis, credit / source https://discourse.roots.io/t/install-piwik-with-trellis/10743/4

matomo.yml

# group_vars/{staging & production}/wordpress_sites.yml
[...]
wordpress_sites:
  example.com:
    site_hosts: [...]
    [...]
    matomo:
      db:
        user: matomo
        host: localhost

# group_vars/{staging & production}/vault.yml
[...]
vault_wordpress_sites:
  example.com:
    env:
      [...]
    matomo:
      db:
        password: XXXXXX

# server.yml
[...]
- { role: matomo, tags: [matomo] }

# roles/matomo/defaults/main.yml
combined_wordpress_sites: "{{ wordpress_sites|combine(vault_wordpress_sites, recursive=True) }}"
addons_dir: "{{ www_root }}/{{ item.key }}/addons"
matomo_db_name: "matomo_{{ env }}"

# roles/matomo/tasks/main.yml
- name: Create addons folder of sites
  file:
    path: "{{ addons_dir }}"
    owner: "{{ web_user }}"
    group: "{{ web_group }}"
    mode: 0755
    state: directory
  with_dict: "{{ wordpress_sites }}"

- name: Install Matomo
  composer:
      command: require
      arguments: piwik/piwik
      working_dir: "{{ addons_dir }}"
  with_dict: "{{ wordpress_sites }}"
  tags: matomo

- name: Create databases for Matomo
  mysql_db:
    name: "{{ matomo_db_name }}"
    state: present
    login_host: "{{ site_env.db_host }}"
    login_user: "{{ mysql_root_user }}"
    login_password: "{{ mysql_root_password }}"
  with_dict: "{{ wordpress_sites }}"
  tags: matomo

- name: Create/assign database user to db and grant permissions
  mysql_user:
    name: "{{ item.value.matomo.db.user }}"
    password: "{{ item.value.matomo.db.password }}"
    host: "{{ site_env.db_user_host }}"
    append_privs: yes
    priv: "{{ matomo_db_name }}.*:ALL"
    state: present
    login_host: "{{ site_env.db_host }}"
    login_user: "{{ mysql_root_user }}"
    login_password: "{{ mysql_root_password }}"
  with_dict: "{{ combined_wordpress_sites }}"
  tags: matomo

- name: Change addons owner to web_user
  file:
    path: "{{ addons_dir }}"
    owner: "{{ web_user }}"
    group: "{{ web_group }}"
    state: directory
    recurse: yes
  with_dict: "{{ wordpress_sites }}"
  tags: matomo


- name: Explain next steps
  debug:
    msg: |
      If necessary, set up Matomo as follows.
      1) Deploy
      2) Point your browser to {{ wordpress_env_defaults.wp_home }}/analytics
      3) Proceed with the form using following credentials:
         Host:              {{ site_env.db_user_host }}
         Database User:     {{ item.value.matomo.db.user }}
         Database Password: {{ item.value.matomo.db.password }}
         Database:          {{ matomo_db_name }}
         Table Prefix:      none
         Set up your admin user.
      4) Log in to Matomo.
      5) Under Administration/System/Geolocation, download and activate the GeoIP 2 database.
  with_dict: "{{ combined_wordpress_sites }}"
  tags: matomo

# roles/deploy/defaults/main.yml
[...]
addons_dir: "{{ project_root }}/addons"
[...]
deploy_share_after:
  - "{{ playbook_dir }}/deploy-hooks/share-after.yml"

# deploy-hooks/share-after.yml
- name: Create symlink to Matomo
  file:
    path: "{{ deploy_helper.new_release_path }}/web/analytics"
    src: "{{ addons_dir }}/vendor/piwik/piwik"
    state: link
  with_items: "{{ wordpress_sites }}"

IMPORTANT

The current Matomo build doesn't seem to play nice with regards to the composer setup, had to revert to v3.8.0 and install the JShrink dependency explicitely beforehand...

# roles/matomo/tasks/main.yml

- name: Install JShrink dependency (Matomo installation fails otherwise...)
  composer:
    command: require
    arguments: tedivm/jshrink ~0.5.1
    working_dir: "{{ addons_dir }}"
  with_dict: "{{ wordpress_sites }}"

- name: Install Matomo
  composer:
    command: require
    arguments: piwik/piwik 3.8.0
    working_dir: "{{ addons_dir }}"
  with_dict: "{{ wordpress_sites }}"
  tags: matomo