configure Loki for local development (#26997)

* configure Loki for local development

Extend our docker compose setup to include Loki (along with promtail), scraping `logs/*.log` to ingest development logs as well as all logs from running containers.

While we're in here, teach Prometheus to scrape metrics from Docker containers too.

* tweak promtail labels

* document extra services in config.mk

* update home.json

server/build/docker/prometheus.yml

@@ -6,3 +6,13 @@ scrape_configs:
   - job_name: 'mattermost'
     static_configs:
       - targets: ['host.docker.internal:8067']
+  - job_name: fmetrics_scrape
+    docker_sd_configs:
+      - host: unix:///var/run/docker.sock
+        refresh_interval: 5s
+    relabel_configs:
+      - source_labels: ['__meta_docker_container_name']
+        regex: '/(.*)'
+        target_label: 'container'
+      - source_labels: ['__meta_docker_container_log_stream']
+        target_label: 'logstream'