Add DogStatsD metrics emission

[revied]

Summary

Adds optional DogStatsD metrics emission for Version Guard scan and inventory metrics.

• Adds a MetricsEmitter abstraction with a no-op default and DogStatsD implementation
• Emits version_guard.findings.*, version_guard.compliance_percentage, version_guard.detection.duration_ms, version_guard.inventory.fetch, version_guard.inventory.resources, and version_guard.scan.completed
• Wires metrics into inventory fetch and detection summary activities without requiring Datadog for local/OSS users
• Adds METRICS_BACKEND, DOGSTATSD_ADDR, and METRICS_TAGS config plus docs/env examples

Testing

• go test ./cmd/server ./pkg/emitters ./pkg/workflow/detection
• make lint
• make test
• make build-all
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 68.26923% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/emitters/metrics.go	68.18%	18 Missing and 3 partials ⚠️
pkg/workflow/detection/activities.go	67.56%	9 Missing and 3 partials ⚠️

Files with missing lines	Coverage Δ
pkg/workflow/detection/workflow.go	84.52% <100.00%> (ø)
pkg/workflow/detection/activities.go	84.06% <67.56%> (ø)
pkg/emitters/metrics.go	68.18% <68.18%> (ø)

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[revied]

Closing this approach; we only want to enable Temporal SDK metrics rather than add custom Version Guard metrics emission.