telemetry: include user decisions in tool call logs (#966)

Add the user's decision (accept, reject, modify) to tool call telemetry to better understand user intent. The decision provides crucial context to the `success` metric, as a user can reject a call that would have succeeded or accept one that fails. 

Also prettify the arguments json.

Example: 
![image](https://github.com/user-attachments/assets/251cb9fc-ceaa-4cdd-929c-8de47031aca8)

#750

docs/core/telemetry.md

@@ -288,6 +288,7 @@ These are timestamped records of specific events.
     - `function_args`
     - `duration_ms`
     - `success` (boolean)
+    - `decision` (string: "accept", "reject", or "modify", optional)
     - `error` (optional)
     - `error_type` (optional)
 
@@ -332,11 +333,13 @@ These are numerical measurements of behavior over time.
   - **Attributes**:
     - `function_name`
     - `success` (boolean)
+    - `decision` (string: "accept", "reject", or "modify", optional)
 
 - `gemini_cli.tool.call.latency` (Histogram, ms): Measures tool call latency.
 
   - **Attributes**:
     - `function_name`
+    - `decision` (string: "accept", "reject", or "modify", optional)
 
 - `gemini_cli.api.request.count` (Counter, Int): Counts all API requests.