feat: add exclude-list for logs in /var/log

[lucabello]

Issue

Closes #48.

Solution

Grafana Agent scrapes all the log files under /var/log via a scrape job that defines __path__: /var/log/**/*.log. The scrape job definition allows defining a __path_exclude__ key at the same level, allowing a Glob pattern matching to exclude files from the scrape.

This PR allows users to set that __path_exclude__ key with a config option.

Testing instructions

1. Deploy Grafana Agent (with `juju config agent exclude_files="some/glob") + Zookeeper on machine, Loki + Traefik + Grafana (to use as Loki UI) on k8s.
2. Create a file under the excluded path.
3. Verify the log doesn't appear in Loki.