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/telemetry/metrics.ts

@@ -20,15 +20,23 @@ import {
   METRIC_API_REQUEST_LATENCY,
   METRIC_TOKEN_USAGE,
   METRIC_SESSION_COUNT,
+  METRIC_FILE_OPERATION_COUNT,
 } from './constants.js';
 import { Config } from '../config/config.js';
 
+export enum FileOperation {
+  CREATE = 'create',
+  READ = 'read',
+  UPDATE = 'update',
+}
+
 let cliMeter: Meter | undefined;
 let toolCallCounter: Counter | undefined;
 let toolCallLatencyHistogram: Histogram | undefined;
 let apiRequestCounter: Counter | undefined;
 let apiRequestLatencyHistogram: Histogram | undefined;
 let tokenUsageCounter: Counter | undefined;
+let fileOperationCounter: Counter | undefined;
 let isMetricsInitialized = false;
 
 function getCommonAttributes(config: Config): Attributes {
@@ -75,7 +83,10 @@ export function initializeMetrics(config: Config): void {
     description: 'Counts the total number of tokens used.',
     valueType: ValueType.INT,
   });
-
+  fileOperationCounter = meter.createCounter(METRIC_FILE_OPERATION_COUNT, {
+    description: 'Counts file operations (create, read, update).',
+    valueType: ValueType.INT,
+  });
   const sessionCounter = meter.createCounter(METRIC_SESSION_COUNT, {
     description: 'Count of CLI sessions started.',
     valueType: ValueType.INT,
@@ -171,3 +182,21 @@ export function recordApiErrorMetrics(
     model,
   });
 }
+
+export function recordFileOperationMetric(
+  config: Config,
+  operation: FileOperation,
+  lines?: number,
+  mimetype?: string,
+  extension?: string,
+): void {
+  if (!fileOperationCounter || !isMetricsInitialized) return;
+  const attributes: Attributes = {
+    ...getCommonAttributes(config),
+    operation,
+  };
+  if (lines !== undefined) attributes.lines = lines;
+  if (mimetype !== undefined) attributes.mimetype = mimetype;
+  if (extension !== undefined) attributes.extension = extension;
+  fileOperationCounter.add(1, attributes);
+}