Enable Loki monitoring

Enable monitoring configuration in the Loki chart: serviceMonitor (so Loki pods are discovered by Prometheus), rules and dashboards

The loki-gateway endpoints have to be dropped from monitoring (401 response) as we use HTTP basicAuth and the chart doesn't handle this case (https://github.com/grafana/loki/issues/14141)

The default scrape interval of 15s can't be changed as the default Loki rules use a 1m interval for rate and other functions and we need at least 3 data points for it to properly work

Part of: #1701 (closed)