Add docs for foreach

docs/sources/reference/config-blocks/foreach.md

@@ -0,0 +1,198 @@
+---
+canonical: https://grafana.com/docs/alloy/latest/reference/stdlib/foreach/
+description: Learn about foreach
+menuTitle: foreach
+title: foreach
+---
+
+<span class="badge docs-labels__stage docs-labels__item">Experimental</span>
+
+# foreach
+
+{{< docs/shared lookup="stability/experimental_feature.md" source="alloy" version="<ALLOY_VERSION>" >}}
+
+The `foreach` block runs a separate pipeline for each item inside a list.
+
+## Usage
+
+```alloy
+foreach "<LABEL>" {
+  collection = [...]
+  var        = "<VAR_NAME>"
+  template {
+    ...
+  }
+}
+```
+
+## Arguments
+
+The following arguments are supported:
+
+Name             | Type        | Description                                                           | Default | Required
+-----------------|-------------|-----------------------------------------------------------------------|---------|---------
+`collection`     | `list(any)` | A list of items to loop over.                                         |         | yes
+`var`            | `string`    | Name of the variable referring to the current item in the collection. |         | yes
+`enable_metrics` | `bool`      | Whether to expose debug metrics in the {{< param "PRODUCT_NAME" >}} `/metrics` endpoint.       | `false` | no
+
+The items in the `collection` list can be of any type [type][types], such as a bool, a string, a list, or a map.
+
+{{< admonition type="warning" >}}
+Setting `enable_metrics` to `true` when `collection` has lots of elements may cause a large number of metrics to appear on the {{< param "PRODUCT_NAME" >}} `/metric` endpoint.
+{{< /admonition >}}
+
+[types]: ../../../get-started/configuration-syntax/expressions/types_and_values
+
+## Blocks
+
+The following blocks are supported inside the definition of `foreach`:
+
+Hierarchy | Block        | Description                  | Required
+----------|--------------|------------------------------|---------
+template  | [template][] | A component pipeline to run. | yes
+
+[template]: #template-block
+
+### template
+
+The `template` block contains the definition of {{< param "PRODUCT_NAME" >}} components which will be ran for every item in the collection.
+The contents of the block look like a normal {{< param "PRODUCT_NAME" >}} configuration file,
+except that you can use the keyword defined in `var` to refer to the current item in the collection.
+
+Components inside the `template` block can use exports of components defined outside of the `foreach` block.
+However, components outside of the `foreach` cannot use exports from components defined inside the `template` block of a `foreach`.
+
+## Example
+
+The following example shows you how to run Prometheus exporters dynamically on service discovery targets.
+
+`prometheus.exporter.*` components often require the address of one particular instance being monitored.
+For example, `prometheus.exporter.redis` has a `redis_addr` attribute for the Redis instance under observation.
+On the other hand, `discovery.*` components such as `discovery.kubernetes` output a list of targets such as this:
+
+{{< collapse title="Example targets output by discovery.kubernetes" >}}
+```json
+[
+    {
+    __address__                                          = "10.42.0.16:5432",
+    __meta_kubernetes_namespace                          = "ns1",
+    __meta_kubernetes_pod_container_id                   = "containerd://96b77d035d0bbe27bb173d8fc0c56d21965892a50e4e6eab9f6cffdb90b275fb",
+    __meta_kubernetes_pod_container_image                = "postgres:bullseye",
+    __meta_kubernetes_pod_container_init                 = "false",
+    __meta_kubernetes_pod_container_name                 = "pgcont",
+    __meta_kubernetes_pod_container_port_name            = "pg-db",
+    __meta_kubernetes_pod_container_port_number          = "5432",
+    __meta_kubernetes_pod_container_port_protocol        = "TCP",
+    __meta_kubernetes_pod_controller_kind                = "ReplicaSet",
+    __meta_kubernetes_pod_controller_name                = "postgres-db-cd54547b9",
+    __meta_kubernetes_pod_host_ip                        = "172.25.0.2",
+    __meta_kubernetes_pod_ip                             = "10.42.0.16",
+    __meta_kubernetes_pod_label_name                     = "postgres-db",
+    __meta_kubernetes_pod_label_pod_template_hash        = "cd54547b9",
+    __meta_kubernetes_pod_labelpresent_name              = "true",
+    __meta_kubernetes_pod_labelpresent_pod_template_hash = "true",
+    __meta_kubernetes_pod_name                           = "postgres-db-cd54547b9-4zpds",
+    __meta_kubernetes_pod_node_name                      = "k3d-asserts-test-server-0",
+    __meta_kubernetes_pod_phase                          = "Running",
+    __meta_kubernetes_pod_ready                          = "true",
+    __meta_kubernetes_pod_uid                            = "7cdcacdc-4a2d-460a-b1fb-6340700c4cac",
+    },
+    {
+    __address__                                          = "10.42.0.20:6379",
+    __meta_kubernetes_namespace                          = "ns1",
+    __meta_kubernetes_pod_container_id                   = "containerd://68f2f0eacd880eb4a141d833aafc1f297f7d9bdf00f4c787f9fcc964a039d278",
+    __meta_kubernetes_pod_container_image                = "redis:latest",
+    __meta_kubernetes_pod_container_init                 = "false",
+    __meta_kubernetes_pod_container_name                 = "redis-cont",
+    __meta_kubernetes_pod_container_port_name            = "redis-db",
+    __meta_kubernetes_pod_container_port_number          = "6379",
+    __meta_kubernetes_pod_container_port_protocol        = "TCP",
+    __meta_kubernetes_pod_controller_kind                = "ReplicaSet",
+    __meta_kubernetes_pod_controller_name                = "redis-db-778b66cb7d",
+    __meta_kubernetes_pod_host_ip                        = "172.25.0.2",
+    __meta_kubernetes_pod_ip                             = "10.42.0.20",
+    __meta_kubernetes_pod_label_name                     = "redis-db",
+    __meta_kubernetes_pod_label_pod_template_hash        = "778b66cb7d",
+    __meta_kubernetes_pod_labelpresent_name              = "true",
+    __meta_kubernetes_pod_labelpresent_pod_template_hash = "true",
+    __meta_kubernetes_pod_name                           = "redis-db-778b66cb7d-wxmf6",
+    __meta_kubernetes_pod_node_name                      = "k3d-asserts-test-server-0",
+    __meta_kubernetes_pod_phase                          = "Running",
+    __meta_kubernetes_pod_ready                          = "true",
+    __meta_kubernetes_pod_uid                            = "ae74e400-8eda-4b02-b4c8-669473fb001b",
+    }
+]
+```
+{{< /collapse >}}
+
+You can use a `foreach` to loop over each target and start a separate component pipeline for it.
+The following example configuration shows how a `prometheus.exporter.redis` instance is started for each Redis instance discoverd by `discovery.kubernetes`.
+Additional Kubernetes labels from `discovery.kubernetes` are also added to the metrics created by `prometheus.exporter.redis`.
+
+```alloy
+discovery.kubernetes "default" {
+    role = "pod"
+}
+
+discovery.relabel "redis" {
+    targets = discovery.kubernetes.default.targets
+
+    // Remove all targets except the Redis ones.
+    rule {
+        source_labels = ["__meta_kubernetes_pod_container_name"]
+        regex         = "redis-cont"
+        action        = "keep"
+    }
+}
+
+// Collect metrics for each Redis instance.
+foreach "redis" {
+    collection = discovery.relabel.redis.output
+    var        = "each"
+
+    template {
+        prometheus.exporter.redis "default" {
+            // This is the "__address__" label from discovery.kubernetes.
+            redis_addr = each["__address__"]
+        }
+
+        prometheus.scrape "default" {
+            targets    = prometheus.exporter.redis.default.targets
+            forward_to = [prometheus.relabel.default.receiver]
+        }
+
+        // Add labels from discovery.kubernetes.
+        prometheus.relabel "default" {
+            rule {
+                replacement  = each["__meta_kubernetes_namespace"]
+                target_label = "k8s_namespace"
+                action       = "replace"
+            }
+
+            rule {
+                replacement  = each["__meta_kubernetes_pod_container_name"]
+                target_label = "k8s_pod_container_name"
+                action       = "replace"
+            }
+
+            forward_to = [prometheus.remote_write.mimir.receiver]
+        }
+    }
+}
+
+prometheus.remote_write "mimir" {
+    endpoint {
+        url = "https://prometheus-xxx.grafana.net/api/prom/push"
+
+        basic_auth {
+            username = sys.env("<PROMETHEUS_USERNAME>")
+            password = sys.env("<GRAFANA_CLOUD_API_KEY>")
+        }
+    }
+}
+```
+
+Replace the following:
+
+* _`<PROMETHEUS_USERNAME>`_: Your Prometheus username.
+* _`<GRAFANA_CLOUD_API_KEY>`_: Your Grafana Cloud API key.

docs/sources/reference/stdlib/array.md

@@ -35,9 +35,7 @@ Elements within the list can be any type.
 
 ## array.combine_maps
 
-> **EXPERIMENTAL**: This is an [experimental][] feature.
-> Experimental features are subject to frequent breaking changes, and may be removed with no equivalent replacement.
-> The `stability.level` flag must be set to `experimental` to use the feature.
+{{< docs/shared lookup="stability/experimental_feature.md" source="alloy" version="<ALLOY_VERSION>" >}}
 
 The `array.combine_maps` function allows you to join two arrays of maps if certain keys have matching values in both maps. It's particularly useful when combining labels of targets coming from different `prometheus.discovery.*` or `prometheus.exporter.*` components.
 It takes three arguments:

docs/sources/shared/stability/experimental_feature.md

@@ -0,0 +1,11 @@
+---
+canonical: https://grafana.com/docs/alloy/latest/shared/stability/experimental_feature/
+description: Shared content, experimental
+headless: true
+---
+
+> **EXPERIMENTAL**: This is an [experimental][] feature.
+> Experimental features are subject to frequent breaking changes, and may be removed with no equivalent replacement.
+> The `stability.level` flag must be set to `experimental` to use the feature.
+
+[experimental]: https://grafana.com/docs/release-life-cycle/