ExecStartPre and EnvironmentFile settings to system unit file

[fabiiw05]

I propose adding the capability to configure ExecStartPre and EnvironmentFile in the system unit file. This enhancement allows us to dynamically define variables before running the Grafana Agent, enabling their use in config.yaml.

For example, this change makes it possible to fetch EC2 tags and use them as labels for metrics.

Below is an example script that retrieves the instance ID, queries for the project tag associated with that EC2 instance, and writes it to an environment file. This tag is then used as an external label in metrics configuration.

#! /bin/sh

export INSTANCE_ID=$(curl -s http://169.254.169.254/latest/meta-data/instance-id)
ENV_FILE=/var/lib/grafana-agent/aws-env
AWS_PROJECT_TAG=$(aws ec2 describe-instances --region ap-northeast-1 --instance-ids "${INSTANCE_ID}" --query 'Reservations[0].Instances[0].Tags[?Key==`Project`]|[0].Value' --output text)

echo AWS_PROJECT_TAG="${AWS_PROJECT_TAG}" > ${ENV_FILE}

This environment variable is then referenced in the config.yaml for the Grafana Agent as shown below:

metrics:
  global:
    external_labels:
      project: ${AWS_PROJECT_TAG}

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[gardar]

While I don't see anything wrong with adding this I wonder if it's perhaps too niche to be special variables?
Perhaps it would be more useful to add a variable grafana_agent_service_extra which accepts arbitrary options to the systemd unit?

[fabiiw05]

I think that's a great idea. I was also considering setting AmbientCapabilities and CapabilityBoundingSet for logging purposes. Thank you for your suggestion. I'll look into making that adjustment.

[fabiiw05]

I've updated the implementation. To ensure the functionality aligns with our expectations, I defined the following variables and tested the behavior:

grafana_agent_service_extra:
  CapabilityBoundingSet: CAP_DAC_READ_SEARCH
  AmbientCapabilities: CAP_DAC_READ_SEARCH
  EnvironmentFile: /var/lib/grafana-agent/extra-service.env
  ExecStartPre: |-
    /etc/grafana-agent/pre_exec.sh \
          --option1 value1 \
          --option2 value2

[gardar]

LGTM!