feat: whitelist Cloudflare metric hosts

[ElioTohm]

Description

Adds optional host filtering for all Cloudflare exporter metrics that expose a host label. The whitelist is loaded from an externally managed ConfigMap mounted as hosts.yaml and reloaded before each scrape.

Type of Change

• New feature (non-breaking when disabled)
• Documentation update

Testing

• Added focused whitelist parsing, filtering, reload, synchronization, and stale-series tests
• go test ./... -count=1
• go test ./... -race -count=1
• helm lint
• Helm render assertions
• git diff --check

Configuration

Set hostWhitelist.configMapName in Helm values. The external ConfigMap must contain:

hosts:
  - api.example.com
  - www.example.com

The ConfigMap is mounted without subPath so Kubernetes updates are observed automatically.

Additional Notes

The fork retains the design and implementation plan commits alongside the feature commits.

[chaosbox]

Could we simplify host_whitelist.go by decoding the file into a typed struct instead of manually walking yaml.Node?

If the accepted format is just:

type HostWhitelist struct {
Hosts []string yaml:"hosts" json:"hosts"
}

then yaml.Unmarshal can handle the parsing/validation, and this file can focus on reload behavior and exact host matching. The current manual schema checks make the implementation harder to follow and more brittle than it needs to be.