Add telemetry command and refactor telemetry settings (#1060)

#750 

### Telemetry Settings
Refactors telemetry configuration to use a nested `telemetry` object in `settings.json`, for example:

```json
{
  "telemetry": {
    "enabled": true,
    "target": "gcp"
    "log-prompts": "true"
  },
  "sandbox": false
}
```

The above includes
- Centralized telemetry settings under a `telemetry` object in `settings.json`.
- CLI flags for the `gemini` command to override all telemetry sub-settings:
    - `--telemetry` / `--no-telemetry`
    - `--telemetry-target <local|gcp>`
    - `--telemetry-otlp-endpoint <URL>`
    - `--telemetry-log-prompts` / `--no-telemetry-log-prompts`
- Updates `packages/cli/src/config/config.ts` and `packages/core/src/config/config.ts` to read from the new settings structure and respect the new CLI flags.
- Modifies `scripts/handle-telemetry.js`, `scripts/local_telemetry.js`, and `scripts/telemetry_utils.js` to align with the new settings structure.
- Updates `docs/core/telemetry.md` to reflect the new settings structure, CLI flags, and order of precedence.
- Renames `logUserPromptsEnabled` to `logPrompts` for brevity.

### `npm run telemetry`

Add a new `npm run telemetry` command that uses `scripts/telemetry.js`, automates the entire process of setting up a local and GCP telemetry pipelines, including configuring the necessary settings in the `.gemini/settings.json` workspace file and installing required binaries (e.g. `otelcol-contrib`).

---
```shell
$ npm run telemetry -- --target=gcp

> gemini-cli@0.1.0 telemetry
> node scripts/telemetry.js --target=gcp

⚙️  Using command-line target: gcp
🚀 Running telemetry script for target: gcp.
✨ Starting Local Telemetry Exporter for Google Cloud ✨
⚙️  Enabled telemetry in workspace settings.
🔧 Set telemetry OTLP endpoint to http://localhost:4317.
🎯 Set telemetry target to gcp.
✅ Workspace settings updated.
✅ Using Google Cloud Project ID: foo-bar

🔑 Please ensure you are authenticated with Google Cloud:
  - Run `gcloud auth application-default login` OR ensure `GOOGLE_APPLICATION_CREDENTIALS` environment variable points to a valid service account key.
  - The account needs "Cloud Trace Agent", "Monitoring Metric Writer", and "Logs Writer" roles.
✅ otelcol-contrib already exists at /Users/jerop/github/gemini-cli/.gemini/otel/bin/otelcol-contrib
🧹 Cleaning up old processes and logs...
✅ Deleted old GCP collector log.
📄 Wrote OTEL collector config to /Users/jerop/github/gemini-cli/.gemini/otel/collector-gcp.yaml
🚀 Starting OTEL collector for GCP... Logs: /Users/jerop/github/gemini-cli/.gemini/otel/collector-gcp.log
⏳ Waiting for OTEL collector to start (PID: 17013)...
✅ OTEL collector started successfully on port 4317.

✨ Local OTEL collector for GCP is running.

🚀 To send telemetry, run the Gemini CLI in a separate terminal window.

📄 Collector logs are being written to: /Users/jerop/github/gemini-cli/.gemini/otel/collector-gcp.log

📊 View your telemetry data in Google Cloud Console:
   - Logs: https://console.cloud.google.com/logs/query;query=logName%3D%22projects%2Ffoo-bar%2Flogs%2Fgemini_cli%22?project=foo-bar
   - Metrics: https://console.cloud.google.com/monitoring/metrics-explorer?project=foo-bar
   - Traces: https://console.cloud.google.com/traces/list?project=foo-bar

Press Ctrl+C to exit.
^C
👋 Shutting down...
⚙️  Disabled telemetry in workspace settings.
🔧 Cleared telemetry OTLP endpoint.
🎯 Cleared telemetry target.
✅ Workspace settings updated.
🛑 Stopping otelcol-contrib (PID: 17013)...
✅ otelcol-contrib stopped.
```

scripts/telemetry_gcp.js

@@ -70,6 +70,7 @@ async function main() {
   const originalSandboxSetting = manageTelemetrySettings(
     true,
     'http://localhost:4317',
+    'gcp',
   );
   registerCleanup(
     () => [collectorProcess].filter((p) => p), // Function to get processes
@@ -80,9 +81,12 @@ async function main() {
   const projectId = process.env.GOOGLE_CLOUD_PROJECT;
   if (!projectId) {
     console.error(
-      '🛑 Error: GOOGLE_CLOUD_PROJECT environment variable is not set.',
+      '🛑 Error: GOOGLE_CLOUD_PROJECT environment variable is not exported.',
     );
-    console.log('Please set it to your Google Cloud Project ID and try again.');
+    console.log(
+      '   Please set it to your Google Cloud Project ID and try again.',
+    );
+    console.log('   `export GOOGLE_CLOUD_PROJECT=your-project-id`');
     process.exit(1);
   }
   console.log(`✅ Using Google Cloud Project ID: ${projectId}`);
@@ -167,13 +171,13 @@ async function main() {
   console.log(`\n📄 Collector logs are being written to: ${OTEL_LOG_FILE}`);
   console.log(`\n📊 View your telemetry data in Google Cloud Console:`);
   console.log(
-    `   - Traces: https://console.cloud.google.com/traces/list?project=${projectId}`,
+    `   - Logs: https://console.cloud.google.com/logs/query;query=logName%3D%22projects%2F${projectId}%2Flogs%2Fgemini_cli%22?project=${projectId}`,
   );
   console.log(
     `   - Metrics: https://console.cloud.google.com/monitoring/metrics-explorer?project=${projectId}`,
   );
   console.log(
-    `   - Logs: https://console.cloud.google.com/logs/query;query=logName%3D%22projects%2F${projectId}%2Flogs%2Fgemini_cli%22?project=${projectId}`,
+    `   - Traces: https://console.cloud.google.com/traces/list?project=${projectId}`,
   );
   console.log(`\nPress Ctrl+C to exit.`);
 }