Add ALERT_DISCORD_IDS label and env variable

[DiogoSantoss]

Related to ObolNetwork#387

Warning

Breaking Change

This PR changes how metrics are sent to Obol. After this update, you will no longer have to keep stashing and unstashing changes on prometheus/prometheus.yml every update. It will now be generated at runtime by ./prometheus/run.sh, and the required variables will be injected into the file from your .env. The steps you need to take to handle this change are described below.

1. If you currently have a locally modified prometheus/prometheus.yml, make a backup copy of it outside of the repository before upgrading (just in case). Ensure you record the value of PROM_REMOTE_WRITE_TOKEN from the authorization.credentials section of that file.

2. This token must now be provided via your .env file instead of being defined in prometheus.yml. The variable PROM_REMOTE_WRITE_TOKEN is present (commented out) in all .env.sample.* files. To configure it, copy the variable to your own .env, uncomment it and set your token, for example:

PROM_REMOTE_WRITE_TOKEN=obolH7d...

3. We are now adding a dedicated way to add discord IDs to your deployment, allowing you to be @'d directly by our Obol Agent if there is an issue with your node. Add (uncomment) ALERT_DISCORD_IDS= in your .env, and specify one or more (with comma separation) IDs. To get the ID that corresponds to your discord account; Enable developer mode on discord with User Settings > Advanced. Then right click on a user's profile picture or name and select Copy ID to get a unique 18-digit number that represents their account. If you previously changed your CHARON_NICKNAME to your discord ID, you can set that back to a human friendly name for your node.

4. If you have any other custom modifications to your original prometheus.yml, compare it against prometheus.yml.example, which is now used as the base template for generating the final configuration. Any additional fields or customizations should be added to prometheus.yml.example.

5. With your environment variables set, and any other modifications ported to the prometheus.yml.example file; run docker compose up -d as normal. Confirm that your metrics are being received on the Obol Grafana. Once you have verified that the new Prometheus setup is working as expected, you can safely delete your backup of the old prometheus.yml.

[OisinKyne]

Needs .env.sample.network commented out sample env var

# To get Alerted with Obol Agent monitoring on Discord, specify your Discord ID(s) below.
# Enable developer mode on discord with User Settings > Advanced. 
# Then right click on a user's profile picture or name and select Copy ID to get a unique 18-digit number that represents their account. 
# Specify multiple discord IDs using comma separation. (e.g. `DISCORD_IDS=123456789098765432,098765432123456789`)
#DISCORD_IDS=""

[OisinKyne]

Small changes