feat: scrape metrics from containers via Docker label discovery

Containers opt in with prometheus.io/scrape=true and optionally set
prometheus.io/port, prometheus.io/path, and prometheus.io/auth=basic.

config.alloy.tmpl

@@ -55,6 +55,90 @@ prometheus.remote_write "prometheus" {
     }
   }
 }
+
+// Scrape Prometheus metrics from other containers on this host.
+// Containers opt in via Docker labels:
+//   prometheus.io/scrape=true        required: enable scraping
+//   prometheus.io/port=9090          optional: port exposing /metrics (defaults to first exposed port)
+//   prometheus.io/path=/metrics      optional: path to metrics endpoint (default: /metrics)
+//   prometheus.io/auth=basic         optional: use basic auth with the shared basic_auth secret
+//
+// Uses docker_gwbridge — the host-local bridge network Docker attaches all
+// Swarm containers to for outbound connectivity. Alloy can reach any container
+// on the same host via this network without needing to join each stack's
+// overlay network.
+discovery.docker "containers" {
+  host                = "unix:///var/run/docker.sock"
+  match_first_network = false
+}
+
+discovery.relabel "metrics" {
+  targets = discovery.docker.containers.targets
+
+  rule {
+    source_labels = ["__meta_docker_network_name"]
+    regex         = "docker_gwbridge"
+    action        = "keep"
+  }
+
+  rule {
+    source_labels = ["__meta_docker_container_label_prometheus_io_scrape"]
+    regex         = "true"
+    action        = "keep"
+  }
+
+  rule {
+    source_labels = ["__address__", "__meta_docker_container_label_prometheus_io_port"]
+    regex         = `(.+):\d+;(\d+)`
+    target_label  = "__address__"
+    replacement   = "$1:$2"
+  }
+
+  rule {
+    source_labels = ["__meta_docker_container_label_prometheus_io_path"]
+    regex         = `(.+)`
+    target_label  = "__metrics_path__"
+  }
+
+  rule {
+    source_labels = ["__meta_docker_container_label_com_docker_swarm_service_name"]
+    target_label  = "job"
+  }
+}
+
+discovery.relabel "metrics_noauth" {
+  targets = discovery.relabel.metrics.output
+  rule {
+    source_labels = ["__meta_docker_container_label_prometheus_io_auth"]
+    regex         = "^$"
+    action        = "keep"
+  }
+}
+
+discovery.relabel "metrics_basicauth" {
+  targets = discovery.relabel.metrics.output
+  rule {
+    source_labels = ["__meta_docker_container_label_prometheus_io_auth"]
+    regex         = "basic"
+    action        = "keep"
+  }
+}
+
+prometheus.scrape "containers" {
+  scrape_interval = "120s"
+  targets         = discovery.relabel.metrics_noauth.output
+  forward_to      = [prometheus.remote_write.prometheus.receiver]
+}
+
+prometheus.scrape "containers_basicauth" {
+  scrape_interval = "120s"
+  targets         = discovery.relabel.metrics_basicauth.output
+  forward_to      = [prometheus.remote_write.prometheus.receiver]
+  basic_auth {
+    username = "admin"
+    password = "{{ secret "basic_auth" }}"
+  }
+}
 {{ end }}
 
 {{ if ne (env "LOKI_PUSH_URL") "" }}