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/utils/fileUtils.ts

@@ -16,6 +16,16 @@ const MAX_LINE_LENGTH_TEXT_FILE = 2000;
 // Default values for encoding and separator format
 export const DEFAULT_ENCODING: BufferEncoding = 'utf-8';
 
+/**
+ * Looks up the specific MIME type for a file path.
+ * @param filePath Path to the file.
+ * @returns The specific MIME type string (e.g., 'text/python', 'application/javascript') or undefined if not found or ambiguous.
+ */
+export function getSpecificMimeType(filePath: string): string | undefined {
+  const lookedUpMime = mime.lookup(filePath);
+  return typeof lookedUpMime === 'string' ? lookedUpMime : undefined;
+}
+
 /**
  * Checks if a path is within a given root directory.
  * @param pathToCheck The absolute path to check.