fix: use configured service name for OTEL resources (#11818)

# Which Problems Are Solved

The OTEL service.name resource attribute is hardcoded to "ZITADEL" in
setupOTelSDK, so the Instrumentation.ServiceName config and the
OTEL_SERVICE_NAME environment variable are both ignored. This makes it
impossible to distinguish between multiple Zitadel instances or
co-located services in tracing dashboards and alerting rules.

# How the Problems Are Solved

The hardcoded string is replaced with cfg.ServiceName so the configured
value flows through. An explicit check for the OTEL_SERVICE_NAME
environment variable is added at the top of resourceWithService, giving
it the highest priority per the OpenTelemetry specification.

# Additional Changes

None.

# Additional Context

Closes #11455
Backport to v4

backend/v3/instrumentation/otel.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"os"
 	"time"
 
 	"go.opentelemetry.io/otel"
@@ -79,7 +80,7 @@ func setupOTelSDK(ctx context.Context, cfg Config) (*log.LoggerProvider, Shutdow
 	prop := newPropagator()
 	otel.SetTextMapPropagator(prop)
 
-	resource, err := resourceWithService("ZITADEL")
+	resource, err := resourceWithService(cfg.ServiceName)
 	if err != nil {
 		return nil, shutdown, err
 	}
@@ -121,6 +122,11 @@ func newPropagator() propagation.TextMapPropagator {
 }
 
 func resourceWithService(serviceName string) (*resource.Resource, error) {
+	// OTEL_SERVICE_NAME takes priority over programmatic config per the
+	// OpenTelemetry specification.
+	if envName := os.Getenv("OTEL_SERVICE_NAME"); envName != "" {
+		serviceName = envName
+	}
 	attributes := []attribute.KeyValue{
 		semconv.ServiceNameKey.String(serviceName),
 	}