chore(release): prepare for v0.3.1 (#145)

## [0.3.1] - 2026-05-29

### 🐛 Bug Fixes

- Set service.instance.id for unique instance label (#144)

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---

**Note:** Please use **Squash Merge** when merging this PR.

Reviewed-on: #145
Co-authored-by: Unbound Releaser <releaser@unbound.se>
Co-committed-by: Unbound Releaser <releaser@unbound.se>

.version

@@ -1,3 +1,3 @@
 {
-  "version": "v0.3.0"
+  "version": "v0.3.1"
 }

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 All notable changes to this project will be documented in this file.
 
+## [0.3.1] - 2026-05-29
+
+### 🐛 Bug Fixes
+
+- Set service.instance.id for unique instance label (#144)
+
 ## [0.3.0] - 2026-05-26
 
 ### 🚀 Features

otel.go

@@ -22,7 +22,17 @@ import (
 // SetupOTelSDK bootstraps the OpenTelemetry pipeline.
 func SetupOTelSDK(ctx context.Context, enabled bool, serviceName, buildVersion, environment string) (func(context.Context) error, error) {
 	if os.Getenv("OTEL_RESOURCE_ATTRIBUTES") == "" {
-		if err := os.Setenv("OTEL_RESOURCE_ATTRIBUTES", fmt.Sprintf("service.name=%s,service.version=%s,service.environment=%s", serviceName, buildVersion, environment)); err != nil {
+		// service.instance.id makes every pod a distinct telemetry resource. The
+		// OTLP→Prometheus exporter maps it to the `instance` label on metrics and
+		// target_info, which keeps multi-replica services from colliding on a
+		// single series and gives joins a unique (job, instance) key. Hostname is
+		// the pod name under Kubernetes; fall back to the service name if it is
+		// unavailable so the attribute is always present.
+		instanceID, err := os.Hostname()
+		if err != nil || instanceID == "" {
+			instanceID = serviceName
+		}
+		if err := os.Setenv("OTEL_RESOURCE_ATTRIBUTES", fmt.Sprintf("service.name=%s,service.version=%s,service.environment=%s,service.instance.id=%s", serviceName, buildVersion, environment, instanceID)); err != nil {
 			return func(context.Context) error {
 				return nil
 			}, err