Add prometheus role

Configured to include job definitions from /etc/prometheus/conf.d/*.yml.

roles/prometheus/README.md

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+Install and configure prometheus.
+
+Job definitions should be placed in /etc/prometheus/conf.d by roles using this one.

roles/prometheus/files/prometheus.yml

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+scrape_config_files:
+  - "conf.d/*.yml"

roles/prometheus/handlers/main.yml

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+- name: reload prometheus
+  service:
+    name: prometheus
+    state: reloaded
+  when: "'handler' not in ansible_skip_tags"

roles/prometheus/tasks/main.yml

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+- name: Install packages
+  package:
+    name:
+      - prometheus
+
+- name: Create directory for prometheus configs
+  file:
+    path: /etc/prometheus/conf.d
+    state: directory
+
+- name: Configure prometheus
+  copy:
+    dest: /etc/prometheus/
+    src: prometheus.yml
+  notify: reload prometheus
+
+- name: Enable prometheus service
+  service:
+    name: prometheus
+    enabled: true
+    state: started