[prometheus-cloudwatch-exporter] add commonLabels support

[firasmosbehi]

Summary

This PR adds commonLabels support to prometheus-cloudwatch-exporter resources and addresses #6247.

Problem

The chart exposes resource-specific label knobs (service.labels, deployment.labels, etc.) but lacks a global/common label mechanism. Users wanting organization-wide labels on all chart resources must duplicate labels per-resource or patch rendered manifests.

Root cause

No top-level value exists to inject shared labels into metadata across all templates.

Fix

• Added commonLabels: {} to values.yaml.
• Wired commonLabels into metadata labels for all chart-rendered resources.
• Included commonLabels in deployment pod template labels.
• Ensured PrometheusRule and ServiceMonitor metadata labels include commonLabels even when chart-specific label maps are empty.
• Bumped chart version from 0.28.1 to 0.28.2.

Validation

• helm lint charts/prometheus-cloudwatch-exporter
• helm template test charts/prometheus-cloudwatch-exporter --set commonLabels.team=platform --set serviceMonitor.enabled=true --set prometheusRule.enabled=true
• Verified repeated team: platform labels across rendered resources.

Compatibility

Backward compatible: commonLabels defaults to {} and does not affect existing installs unless explicitly set.