Allow customizable phone call message template for alert notifications

[smutosey]

What would you like to see!

The current phone call message template is hard-coded in engine/apps/alerts/incident_appearance/renderers/phone_call_renderer.py:

TEMPLATE = (
    "You are invited to check an Alert Group from Grafana OnCall. "
    "Alert via {integration_name} with title {title} triggered {alert_count} times"
)

It would be nice to have this template be customizable. Specifically, I propose the following improvements:

• Customization via UI or Environment Variables: Allow users to modify the message template without changing the source code, either through the Grafana OnCall UI or by setting environment variables.

• Dynamic Placeholders: Continue supporting placeholders such as {integration_name}, {title}, and {alert_count}, so that users can define their own message formats that better fit their workflow. Common or group labels, summary and annotations, maybe.

• Enhanced Flexibility: This change would solve the pain point of being forced to use a fixed message format, enabling teams with different alerting and notification requirements to tailor the communication style to their needs.

Product Area

Alert Flow & Configuration

Anything else to add?

This issue is relevant for Grafana OnCall version 1.14.3 or lower

[github-actions]

The current version of Grafana OnCall, at the time this issue was opened, is v1.14.3. If your issue pertains to an older version of Grafana OnCall, please be sure to list it in the PR description. Thank you 😄!