[datadog_security_monitoring_rule] Prevent API-echoed metric field from causing apply inconsistency

[alvizi-dd]

Summary

• Extends reconcileEmptyQueryFields to handle the deprecated metric field
• The Datadog API derives and echoes back metric from the metrics list even when the user never configured metric. Since metric is Optional (not Computed), the framework plans it as null when not configured. The API echo caused a "provider produced inconsistent result" error after apply.
• Fix: when the prior plan/state had metric = null, restore it to null after reading the API response, regardless of the echoed value.

Test plan

• Verify rules using only metrics (not metric) apply without inconsistency error
• Verify rules using the deprecated metric field still work correctly
• Verify import of existing rules works (defaults to null for unconfigured metric)