[FEATURE] Migrate Grafana Agent to Grafana Alloy

[merhard]

Is your feature request related to a problem? Please describe.
Grafana Agent has been deprecated and will reach EOL November 1, 2025. It has been replaced with a different tool: Grafana Alloy.

Describe the solution you would like to see
The usage of Grafana Agent in PromEx should be migrated to Grafana Alloy instead. There is a migration guide here: https://grafana.com/docs/alloy/latest/set-up/migrate/from-static/

How would you expect this feature to work
I expect it to be an almost drop-in replacement for the current Grafana Agent code. The current Grafana Agent config file will need to be migrated to the Grafana Alloy format. There seems to be a CLI tool that can migrate existing configs: https://grafana.com/docs/alloy/latest/reference/cli/convert/#static

Additional context
https://grafana.com/docs/agent/latest/