feat(helm): Add configurable prometheus metrics annotation sup (#60)

[spbolton]

Description

This PR adds comprehensive Prometheus metrics scraping support to the DotCMS Helm chart, enabling automated discovery and monitoring of DotCMS pods by Prometheus. The implementation provides full configurability while maintaining backward compatibility.

Changes

📊 Prometheus Configuration (values.yaml)

• Added new prometheus configuration section with full control over scraping behavior
• Configurable scrape annotations: scrape, port, and path
• Default values: scrape: "true", port: "8090", path: "/dotmgt/metrics"
• Feature can be completely disabled via prometheus.enabled: false

🏷️ Pod Annotations (statefulset.yaml)

• Conditional Prometheus annotations added to StatefulSet pod template
• Annotations only applied when prometheus.enabled: true
• Standard Prometheus discovery annotations: prometheus.io/scrape, prometheus.io/port, prometheus.io/path

🔧 Helper Functions (_helpers.tpl)

• Created reusable dotcms.version helper function for consistent version handling
• Special case handling: "1.0.0-SNAPSHOT" → "SNAPSHOT" for cleaner version labels
• Extended environment variable rendering to support Kubernetes downward API via FIELD: prefix
• Enhanced secret and field reference handling in environment variable processing

🏃‍♂️ Environment Variables for Metrics Tagging

When Prometheus is enabled, the following environment variables are automatically injected for consistent metric labeling:

• DOT_METRICS_TAG_APP - Application name (e.g., "dotcms")
• DOT_METRICS_TAG_ENV - Environment name (e.g., "prod")
• DOT_METRICS_TAG_VER - Version tag (truncated, e.g., "21.06.14" or "SNAPSHOT")
• DOT_METRICS_TAG_CUST - Customer name (e.g., "dotcms-dev")
• DOT_METRICS_TAG_FULLNAME - Full deployment name (e.g., "dotcms-dotcms-dev-prod")
• DOT_METRICS_TAG_HOSTNAME - Pod hostname with StatefulSet index via Kubernetes downward API

🔄 Kubernetes Integration

• Hostname environment variable uses proper Kubernetes downward API (valueFrom.fieldRef.fieldPath: "metadata.name")
• Provides actual pod names like dotcms-dotcms-dev-prod-0, dotcms-dotcms-dev-prod-1 for StatefulSet replica identification
• Compatible with Kubernetes service discovery patterns

Testing

• ✅ Validated Helm template rendering with Prometheus enabled/disabled
• ✅ Confirmed annotations appear only when prometheus.enabled: true
• ✅ Verified environment variables render correctly with proper values
• ✅ Tested custom annotation values override defaults properly
• ✅ Validated version helper function handles both regular tags and SNAPSHOT case
• ✅ Confirmed downward API field reference syntax for hostname

Example Usage

Enable with defaults:

prometheus:
  enabled: true

Custom configuration:

prometheus:
  enabled: true
  annotations:
    scrape: "false"
    port: "9090" 
    path: "/custom/metrics"

Disable entirely:

prometheus:
  enabled: false

Impact

• Zero breaking changes - Feature is opt-in and disabled by default
• Enhanced observability - Enables automated Prometheus monitoring of DotCMS deployments
• Flexible configuration - All aspects of metrics scraping are customizable
• StatefulSet friendly - Proper hostname tagging for replica identification
• Consistent labeling - Standardized environment variables for metric consistency across deployments

Closes #60

Issue: Add configurable prometheus metrics annotation support to helm chart

[dcolina]

Looks good. Prometheus support to the helm chart.