Add an example that uses environment variables (#698)

Signed-off-by: Pete Wall <[email protected]>

examples/environment-variables/README.md

@@ -0,0 +1,53 @@
+# Environment Variables
+
+In this example, we show how to set environment variables in the Alloy instance and a few examples of how to use them.
+
+This can be extended to set environment variables in any of the Alloy instances.
+
+```yaml
+---
+cluster:
+  name: environment-variables-test
+
+externalServices:
+  prometheus:
+    host: https://prometheus.example.com
+    basicAuth:
+      username: 12345
+      password: "It's a secret to everyone"
+    externalLabelsFrom:
+      company: env("COMPANY")     # label set from environment variable
+      team: env("team")           # label set from environment variable found in the ConfigMap
+  loki:
+    host: https://loki.example.com
+    basicAuth:
+      username: 12345
+      password: "It's a secret to everyone"
+
+metrics:
+  # Environment variable used in a relabeling rule
+  extraMetricRelabelingRules: |-
+    rule {
+      target_label = "region"
+      replacement = env("region")
+    }
+
+alloy:
+  alloy:
+    extraEnv:       # Set environment variables directly
+      - name: COMPANY
+        value: "Widget Co"
+    extraEnvFrom:   # Set environment variables from a ConfigMap
+      - configMapRef:
+        name: team-params
+
+# Deploy the ConfigMap with the environment variables. Typically, this would already exist in your cluster.
+extraObjects:
+  - apiVersion: v1
+    kind: ConfigMap
+    metadata:
+      name: common-params
+    data:
+      team: "Team A"
+      region: "midwest"
+```