[charts/metabase] Allow external managed HPA

[dumitraand]

Purpose

Allow external HPA controllers (e.g., KEDA) to manage autoscaling for Metabase.

When hpa.external: true, the chart skips creating its own HorizontalPodAutoscaler, avoiding conflicts with external autoscalers while still allowing users to set hpa.enabled: true for any related configuration.

Changes

• Add hpa.external flag (default: false for backward compatibility)
• HPA resource is only created when hpa.enabled=true AND hpa.external=false

Testing

# Test 1: HPA disabled (default) - should NOT create HPA
$ helm template test charts/metabase --set hpa.enabled=false | grep "kind: HorizontalPodAutoscaler"
# (no output - correct)

# Test 2: HPA enabled, external=false - should create HPA
$ helm template test charts/metabase --set hpa.enabled=true,hpa.external=false | grep -A5 "kind: HorizontalPodAutoscaler"
kind: HorizontalPodAutoscaler
metadata:
  name: test-metabase
spec:
  scaleTargetRef:
    apiVersion: apps/v1

# Test 3: HPA enabled, hpa.external not passed - should create HPA
$ helm template test charts/metabase --set hpa.enabled=true | grep -A5 "kind: HorizontalPodAutoscaler"
kind: HorizontalPodAutoscaler
metadata:
  name: test-metabase
spec:
  scaleTargetRef:
    apiVersion: apps/v1
   
# Test 4: HPA enabled, external=true - should NOT create HPA (for KEDA etc.)
$ helm template test charts/metabase --set hpa.enabled=true,hpa.external=true | grep "kind: HorizontalPodAutoscaler"
# (no output - correct)

This demonstrates backward compatibility and the new external HPA behavior (test 3).