Add information about 2 additional metrics with note about opentelemetry-instrumentation-google-genai.

Author: mhenc

docs/observability/metrics.md

@@ -28,6 +28,10 @@ When metrics are enabled, ADK automatically instruments the agent's lifecycle, w
 | **`gen_ai.agent.request.size`** | Histogram | The size or complexity of the incoming request sent to the agent. | `gen_ai.agent.name` |
 | **`gen_ai.agent.response.size`** | Histogram | The size or complexity of the final response generated by the agent. | `gen_ai.agent.name` |
 | **`gen_ai.agent.workflow.steps`** | Histogram | Tracks the number of iterative steps or reasoning loops an agent takes to complete a workflow. | `gen_ai.agent.name` |
+| **`gen_ai.client.operation.duration`*** | Histogram | The duration of GenAI client operations (e.g., LLM calls). | `gen_ai.agent.name`, `gen_ai.agent.version`, `error.type` |
+| **`gen_ai.client.token.usage`*** | Histogram | Token consumption for GenAI client operations. | `gen_ai.agent.name`, `gen_ai.agent.version` |
+
+\* **Note:** ADK natively emits `gen_ai.client.operation.duration` and `gen_ai.client.token.usage`. However, if the `opentelemetry-instrumentation-google-genai` package is installed, ADK suppresses its native emission of these metrics to avoid duplicates. In that case, the metrics will lack the agentic attributes (`gen_ai.agent.name`, `gen_ai.agent.version`).
 
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