Default grafana dashboards

[janciesla8818]

This PR provides easy way to view the metrics exposed by MongoosePush at /metrics endpoint in from of Grafana dashboard. The changes include:

• Adding Makefile that is running docker-compose for Grafana and Prometheus, running update_dashboards.sh script.
• Configuring and provisioning two dashboards - MongoosePush Metrics and MongoosePush VM.
• Renaming docker-compose.prometheus.yml to docker-compose.dashboards.yml as the Prometheus container is being run together with Grafana.
• Documentation update in the guides/metrics section.

To run and configure the containers, please refer to the documentation changes.

To test if the metrics are displayed in the dashboards you can run:

MIX_ENV=integration mix do test.env.up

make dashboards

MIX_ENV=integration mix do test

This will run the integration tests and generate some events inside MongoosePush that can be later seen in the dashboard. Additionally you can trigger few push notifications with curl to generate some traffic.

Go to http://127.0.0.1:3000/ (admin/admin) to see the dashboards.

MongoosePush Metrics.json and MongoosePush VM.json are exported from Grafana. When reviewing the dashboards, no need to go in detail over the files as it's better to run the setup and click though the dashboards in Grafana UI to see if everything is displayed correctly.

Parts of the code and inspiration are taken from: https://github.com/cirocosta/sample-grafana (https://ops.tips/blog/initialize-grafana-with-preconfigured-dashboards/)

[aleklisi]

Just one thing: MIX_ENV=integration mix do test.env.up -> MIX_ENV=integration mix do certs.dev, test.env.up besides that: Works on my computer!! Good Job