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

packages/core/src/tools/read-many-files.ts

@@ -14,9 +14,14 @@ import {
   detectFileType,
   processSingleFileContent,
   DEFAULT_ENCODING,
+  getSpecificMimeType,
 } from '../utils/fileUtils.js';
 import { PartListUnion } from '@google/genai';
 import { Config } from '../config/config.js';
+import {
+  recordFileOperationMetric,
+  FileOperation,
+} from '../telemetry/metrics.js';
 
 /**
  * Parameters for the ReadManyFilesTool.
@@ -420,6 +425,18 @@ Use this tool when the user's query implies needing the content of several files
           contentParts.push(fileReadResult.llmContent); // This is a Part for image/pdf
         }
         processedFilesRelativePaths.push(relativePathForDisplay);
+        const lines =
+          typeof fileReadResult.llmContent === 'string'
+            ? fileReadResult.llmContent.split('\n').length
+            : undefined;
+        const mimetype = getSpecificMimeType(filePath);
+        recordFileOperationMetric(
+          this.config,
+          FileOperation.READ,
+          lines,
+          mimetype,
+          path.extname(filePath),
+        );
       }
     }