Add Metricbeat extension (#567)

Co-authored-by: Antoine Cotten <hello@acotten.com>

extensions/metricbeat/Dockerfile

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+ARG ELK_VERSION
+
+FROM docker.elastic.co/beats/metricbeat:${ELK_VERSION}

extensions/metricbeat/README.md

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+# Metricbeat
+
+Metricbeat is a lightweight shipper that you can install on your servers to periodically collect metrics from the
+operating system and from services running on the server. Metricbeat takes the metrics and statistics that it collects
+and ships them to the output that you specify, such as Elasticsearch or Logstash.
+
+## Usage
+
+If you want to include the Metricbeat extension, run Docker Compose from the root of the repository with an additional
+command line argument referencing the `metricbeat-compose.yml` file:
+
+```bash
+$ docker-compose -f docker-compose.yml -f extensions/metricbeat/metricbeat-compose.yml up
+```
+
+## Documentation
+
+[Official Docs](https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-overview.html)
+[Running on Docker](https://www.elastic.co/guide/en/beats/metricbeat/current/running-on-docker.html)
+[Configuration reference](https://www.elastic.co/guide/en/beats/metricbeat/master/metricbeat-reference-yml.html)

extensions/metricbeat/config/metricbeat.yml

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+## Metricbeat configuration
+## https://github.com/elastic/beats/blob/master/deploy/docker/metricbeat.docker.yml
+#
+
+metricbeat.config:
+  modules:
+    path: ${path.config}/modules.d/*.yml
+    # Reload module configs as they change:
+    reload.enabled: false
+
+metricbeat.autodiscover:
+  providers:
+    - type: docker
+      hints.enabled: true
+
+metricbeat.modules:
+- module: docker
+  metricsets:
+    - container
+    - cpu
+    - diskio
+    - healthcheck
+    - info
+    #- image
+    - memory
+    - network
+  hosts: ['unix:///var/run/docker.sock']
+  period: 10s
+  enabled: true
+
+processors:
+  - add_cloud_metadata: ~
+
+output.elasticsearch:
+  hosts: ['http://elasticsearch:9200']
+  username: elastic
+  password: changeme
+
+## HTTP endpoint for health checking
+## https://www.elastic.co/guide/en/beats/metricbeat/master/http-endpoint.html
+#
+
+http.enabled: true
+http.host: 0.0.0.0

extensions/metricbeat/metricbeat-compose.yml

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+version: '3.2'
+
+services:
+  metricbeat:
+    build:
+      context: extensions/metricbeat/
+      args:
+        ELK_VERSION: $ELK_VERSION
+    # Run as 'root' instead of 'metricbeat' (uid 1000) to allow reading
+    # 'docker.sock' and the host's filesystem.
+    user: root
+    command:
+        # Log to stderr.
+      - -e
+        # Disable config file permissions checks. Allows mounting
+        # 'config/metricbeat.yml' even if it's not owned by root.
+        # see: https://www.elastic.co/guide/en/beats/libbeat/master/config-file-permissions.html
+      - --strict.perms=false
+        # Mount point of the host’s filesystem. Required to monitor the host
+        # from within a container.
+      - --system.hostfs=/hostfs
+    volumes:
+      - type: bind
+        source: ./extensions/metricbeat/config/metricbeat.yml
+        target: /usr/share/metricbeat/metricbeat.yml
+        read_only: true
+      - type: bind
+        source: /
+        target: /hostfs
+        read_only: true
+      - type: bind
+        source: /sys/fs/cgroup
+        target: /hostfs/sys/fs/cgroup
+        read_only: true
+      - type: bind
+        source: /proc
+        target: /hostfs/proc
+        read_only: true
+      - type: bind
+        source: /var/run/docker.sock
+        target: /var/run/docker.sock
+        read_only: true
+    networks:
+      - elk
+    depends_on:
+      - elasticsearch