Add file operation telemetry (#1068)

Introduces telemetry for file create, read, and update operations.

This change adds the `gemini_cli.file.operation.count` metric, recorded by the `read-file`, `read-many-files`, and `write-file` tools.

The metric includes the following attributes:
    - `operation` (string: `create`, `read`, `update`): The type of file operation.
    - `lines` (optional, Int): Number of lines in the file.
    - `mimetype` (optional, string): Mimetype of the file.
    - `extension` (optional, string): File extension of the file.

Here is a stacked bar chart of file operations by extension (`js`, `ts`, `md`):
![image](https://github.com/user-attachments/assets/3e8f8ea9-6155-4186-863c-075cc47647c5)

Here is a stacked bar chart of file operations by type (`create`, `read`, `update`):
![image](https://github.com/user-attachments/assets/3fcf491d-31d0-4ba8-80e6-7fd2bd9c7c27)

#750 

cc @allenhutchison as discussed

docs/core/telemetry.md

@@ -273,6 +273,15 @@ These are numerical measurements of behavior over time.
     - `model`
 
 - `gemini_cli.token.usage` (Counter, Int): Counts the number of tokens used.
+
   - **Attributes**:
     - `model`
     - `type` (string: "input", "output", "thought", "cache", or "tool")
+
+- `gemini_cli.file.operation.count` (Counter, Int): Counts file operations.
+
+  - **Attributes**:
+    - `operation` (string: "create", "read", "update"): The type of file operation.
+    - `lines` (optional, Int): Number of lines in the file.
+    - `mimetype` (optional, string): Mimetype of the file.
+    - `extension` (optional, string): File extension of the file.