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 

1 files changed, 1 insertions, 0 deletions

diff --git a/packages/core/src/telemetry/constants.ts b/packages/core/src/telemetry/constants.ts
index bbcdbada..de760205 100644
--- a/packages/core/src/telemetry/constants.ts
+++ b/packages/core/src/telemetry/constants.ts
@@ -19,3 +19,4 @@ export const METRIC_API_REQUEST_COUNT = 'gemini_cli.api.request.count';
 export const METRIC_API_REQUEST_LATENCY = 'gemini_cli.api.request.latency';
 export const METRIC_TOKEN_USAGE = 'gemini_cli.token.usage';
 export const METRIC_SESSION_COUNT = 'gemini_cli.session.count';
+export const METRIC_FILE_OPERATION_COUNT = 'gemini_cli.file.operation.count';