Add support for a prometheus postgres exporter

This commit introduces a new role that downloads and installs the
prometheus community postgres exporter  https://github.com/prometheus-community/postgres_exporter.
A new credential is added to matrix_postgres_additional_databases that
allows the exporter access to the database to gather statistics.
A new dashboard was added to the grafana role, with some refactoring
to enable the dashboard only if the new role is enabled.
I've included some basic instructions for how to enable the role in
the Docs section.

In terms of testing, I've tested enabling the role, and disabling
it to make sure it cleans up the container and systemd role.

roles/matrix-prometheus-postgres-exporter/defaults/main.yml

@@ -0,0 +1,49 @@
+# matrix-prometheus-postgres-exporter is an Prometheus exporter for postgres metrics
+# See: https://github.com/prometheus-community/postgres_exporter
+
+matrix_prometheus_postgres_exporter_enabled: false
+
+matrix_prometheus_postgres_exporter_version: v0.9.0
+matrix_prometheus_postgres_exporter_port: 9187
+
+matrix_prometheus_postgres_exporter_docker_image: "quay.io/prometheuscommunity/postgres-exporter:{{ matrix_prometheus_postgres_exporter_version }}"
+matrix_prometheus_postgres_exporter_docker_image_force_pull: "{{ matrix_prometheus_postgres_exporter_docker_image.endswith(':latest') }}"
+
+# A list of extra arguments to pass to the container
+matrix_prometheus_postgres_exporter_container_extra_arguments: ["-e PG_EXPORTER_AUTO_DISCOVER_DATABASES=true",
+ "-e PG_EXPORTER_WEB_LISTEN_ADDRESS=\":{{matrix_prometheus_postgres_exporter_port}}\"",
+ "-e DATA_SOURCE_NAME=\"postgresql://{{matrix_prometheus_postgres_exporter_database_username}}:{{matrix_prometheus_postgres_exporter_database_password}}@{{matrix_prometheus_postgres_exporter_database_hostname}}:5432/{{matrix_prometheus_postgres_exporter_database_name}}?sslmode=disable\"" ]
+
+# List of systemd services that matrix-prometheus-postgres-exporter.service depends on
+matrix_prometheus_postgres_exporter_systemd_required_services_list: ['docker.service']
+
+# List of systemd services that matrix-prometheus-postgres-exporter.service wants
+matrix_prometheus_postgres_exporter_systemd_wanted_services_list: []
+
+# details for connecting to the database
+matrix_prometheus_postgres_exporter_database_username: 'matrix_prometheus_postgres_exporter'
+matrix_prometheus_postgres_exporter_database_password: 'some-password'
+matrix_prometheus_postgres_exporter_database_hostname: 'matrix-postgres'
+matrix_prometheus_postgres_exporter_database_port: 5432
+matrix_prometheus_postgres_exporter_database_name: 'matrix_prometheus_postgres_exporter'
+
+
+# Controls whether the matrix-prometheus container exposes its HTTP port (tcp/9100 in the container).
+#
+# Takes an "<ip>:<port>" value (e.g. "127.0.0.1:9100"), or empty string to not expose.
+#
+# Official recommendations are to run this container with `--net=host`,
+# but we don't do that, since it:
+# - likely exposes the metrics web server way too publicly (before applying https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1008)
+# - or listens on a loopback interface only (--net=host and 127.0.0.1:9100), which is not reachable from another container (like `matrix-prometheus`)
+#
+# Using `--net=host` and binding to Docker's `matrix` bridge network may be a solution to both,
+# but that's trickier to accomplish and won't necessarily work (hasn't been tested).
+#
+# Not using `--net=host` means that our network statistic reports are likely broken (inaccurate),
+# because node-exporter can't see all interfaces, etc.
+# For now, we'll live with that, until someone develops a better solution.
+matrix_prometheus_postgres_exporter_container_http_host_bind_port: ''
+
+matrix_prometheus_postgres_exporter_dashboard_urls:
+- "https://grafana.com/api/dashboards/9628/revisions/7/download"

roles/matrix-prometheus-postgres-exporter/tasks/init.yml

@@ -0,0 +1,5 @@
+- set_fact:
+    matrix_systemd_services_list: "{{ matrix_systemd_services_list + ['matrix-prometheus-postgres-exporter.service'] }}"
+  when: matrix_prometheus_postgres_exporter_enabled|bool
+
+

roles/matrix-prometheus-postgres-exporter/tasks/main.yml

@@ -0,0 +1,8 @@
+- import_tasks: "{{ role_path }}/tasks/init.yml"
+  tags:
+    - always
+
+- import_tasks: "{{ role_path }}/tasks/setup.yml"
+  tags:
+    - setup-all
+    - setup-prometheus-postgres-exporter

roles/matrix-prometheus-postgres-exporter/tasks/setup.yml

@@ -0,0 +1,54 @@
+---
+
+#
+# Tasks related to setting up matrix-prometheus-postgres-exporter
+#
+
+- name: Ensure matrix-prometheus-postgres-exporter image is pulled
+  docker_image:
+    name: "{{ matrix_prometheus_postgres_exporter_docker_image }}"
+    source: "{{ 'pull' if ansible_version.major > 2 or ansible_version.minor > 7 else omit }}"
+    force_source: "{{ matrix_prometheus_postgres_exporter_docker_image_force_pull if ansible_version.major > 2 or ansible_version.minor >= 8 else omit }}"
+    force: "{{ omit if ansible_version.major > 2 or ansible_version.minor >= 8 else matrix_prometheus_postgres_exporter_docker_image_force_pull }}"
+  when: "matrix_prometheus_postgres_exporter_enabled|bool"
+
+- name: Ensure matrix-prometheus-postgres-exporter.service installed
+  template:
+    src: "{{ role_path }}/templates/systemd/matrix-prometheus-postgres-exporter.service.j2"
+    dest: "{{ matrix_systemd_path }}/matrix-prometheus-postgres-exporter.service"
+    mode: 0644
+  register: matrix_prometheus_postgres_exporter_systemd_service_result
+  when: matrix_prometheus_postgres_exporter_enabled|bool
+
+- name: Ensure systemd reloaded after matrix-prometheus.service installation
+  service:
+    daemon_reload: yes
+  when: "matrix_prometheus_postgres_exporter_enabled|bool and matrix_prometheus_postgres_exporter_systemd_service_result.changed"
+
+#
+# Tasks related to getting rid of matrix-prometheus-postgres-exporter (if it was previously enabled)
+#
+
+- name: Check existence of matrix-prometheus-postgres-exporter service
+  stat:
+    path: "{{ matrix_systemd_path }}/matrix-prometheus-postgres-exporter.service"
+  register: matrix_prometheus_postgres_exporter_service_stat
+
+- name: Ensure matrix-prometheus-postgres-exporter is stopped
+  service:
+    name: matrix-prometheus-postgres-exporter
+    state: stopped
+    daemon_reload: yes
+  register: stopping_result
+  when: "not matrix_prometheus_postgres_exporter_enabled|bool and matrix_prometheus_postgres_exporter_service_stat.stat.exists"
+
+- name: Ensure matrix-prometheus-postgres-exporter.service doesn't exist
+  file:
+    path: "{{ matrix_systemd_path }}/matrix-prometheus-postgres-exporter.service"
+    state: absent
+  when: "not matrix_prometheus_postgres_exporter_enabled|bool and matrix_prometheus_postgres_exporter_service_stat.stat.exists"
+
+- name: Ensure systemd reloaded after matrix-prometheus-postgres-exporter.service removal
+  service:
+    daemon_reload: yes
+  when: "not matrix_prometheus_postgres_exporter_enabled|bool and matrix_prometheus_postgres_exporter_service_stat.stat.exists"

roles/matrix-prometheus-postgres-exporter/templates/systemd/matrix-prometheus-postgres-exporter.service.j2

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+#jinja2: lstrip_blocks: "True"
+[Unit]
+Description=matrix-prometheus-postgres-exporter
+{% for service in matrix_prometheus_postgres_exporter_systemd_required_services_list %}
+Requires={{ service }}
+After={{ service }}
+{% endfor %}
+{% for service in matrix_prometheus_postgres_exporter_systemd_wanted_services_list %}
+Wants={{ service }}
+{% endfor %}
+DefaultDependencies=no
+
+[Service]
+Type=simple
+Environment="HOME={{ matrix_systemd_unit_home_path }}"
+ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill matrix-prometheus-postgres-exporter 2>/dev/null'
+ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-prometheus-postgres-exporter 2>/dev/null'
+
+
+ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-prometheus-postgres-exporter \
+			--log-driver=none \
+			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
+			--read-only \
+			{% for arg in matrix_prometheus_postgres_exporter_container_extra_arguments %}
+			{{ arg }} \
+			{% endfor %}
+			--network={{ matrix_docker_network }} \
+			{% if matrix_prometheus_postgres_exporter_container_http_host_bind_port %}
+			-p {{ matrix_prometheus_postgres_exporter_container_http_host_bind_port }}:{{matrix_prometheus_postgres_exporter_port}} \
+			{% endif %}
+			--pid=host \
+			{{ matrix_prometheus_postgres_exporter_docker_image }} 
+
+ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill matrix-prometheus-postgres-exporter 2>/dev/null'
+ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-prometheus-postgres-exporter 2>/dev/null'
+Restart=always
+RestartSec=30
+SyslogIdentifier=matrix-prometheus-postgres-exporter
+
+[Install]
+WantedBy=multi-user.target