Add anthropic-usage-metrics dataflow (DataDog#22689)

Author: albertlinde

anthropic_usage_and_costs/assets/dataflows.yaml

@@ -2,3 +2,6 @@ provides:
   - id: anthropic-cloud-cost-metrics
     always_on: false
     granular: true
+  - id: anthropic-usage-metrics
+    always_on: true
+    granular: false