[otel-col] Add docker stats receiver

[julianocosta89]

Changes

This PR adds the docker_stats receiver to the demo.
This receiver is meant to be used only when running locally on Docker.

For that to work we need privileged access to docker sock:

    user: 0:0
    volumes:
      - ${DOCKER_SOCK}:/var/run/docker.sock

I've also added Prometheus receiver to get internal Collector Metrics.

Merge Requirements

For new features contributions please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[puckpuck]

This is a great change; it will lead to a few more things being needed, like Grafana dashboards and what we do about similar functionality in a K8s world. I'll create an issue so we can track the follow-up tasks.