feat(agent-o11y): add agent cost synthetic metric

model/genai/metrics.yaml

@@ -420,6 +420,42 @@ groups:
     instrument: counter
     unit: "{token}"
 
+  # ──────────────────────────────────────────────
+  # dash0.gen_ai.usage.cost
+  # ──────────────────────────────────────────────
+  - id: metric.dash0.gen_ai.usage.cost
+    type: metric
+    metric_name: dash0.gen_ai.usage.cost
+    stability: stable
+    brief: >
+      Total USD cost attributed to GenAI spans.
+    note: |
+      `dash0.gen_ai.usage.cost` is a synthetic metric computed on-the-fly from
+      the ingested span data.
+      Each span contributes the value of its `dash0.gen_ai.usage.cost`
+      attribute (USD, float64) to the per-bucket sum. The cost attribute is
+      written by the `dash0genaicost` collector processor from the span's
+      model and token counts.
+
+      Deprecated PromQL alias: `dash0_gen_ai_usage_cost_total`.
+
+      Synthetic metrics do not expose attributes by default. Attributes are
+      materialised through PromQL aggregations.
+    instrument: counter
+    unit: "USD"
+
+  - id: metric.dash0_gen_ai_usage_cost_total
+    type: metric
+    metric_name: dash0_gen_ai_usage_cost_total
+    stability: stable
+    deprecated:
+      reason: renamed
+      renamed_to: dash0.gen_ai.usage.cost
+    brief: >
+      Deprecated Prometheus alias for `dash0.gen_ai.usage.cost`.
+    instrument: counter
+    unit: "USD"
+
   # ──────────────────────────────────────────────
   # dash0.gen_ai.conversations.info
   # ──────────────────────────────────────────────