Add GCP telemetry script (#1033)

Adds a script -  `scripts/telemetry_gcp.js` - to simplify setting up a local OpenTelemetry collector that forwards data to Google Cloud. This is a follow up to the script for local telemetry `scripts/local_telemetry.js` added in #1015.

This script automates downloading necessary binaries, configuring the collector, and updating workspace settings.

Also includes `scripts/telemetry_utils.js` with shared helper functions for telemetry scripts. Will refactor `scripts/local_t elemetry.js` in next steps to use this shared functionality.

Updates `docs/core/telemetry.md` to include:
- A new "Quick Start" section
- Detailed instructions for the new GCP automated script
- Reorganization of existing sections for clarity

#750 

---
```
✨ Starting Local Telemetry Exporter for Google Cloud ✨
⚙️  Enabled telemetry in workspace settings.
🔧 Set telemetry OTLP endpoint to http://localhost:4317.
✅ 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: 65145)...
✅ OTEL collector started successfully on port 4317.

✨ Local OTEL collector for GCP is running.

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

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

Press Ctrl+C to exit.
^C
👋 Shutting down...
⚙️ Disabled telemetry in workspace settings.
🔧 Cleared telemetry OTLP endpoint.
✅ Workspace settings updated.
🛑 Stopping otelcol-contrib (PID: 65145)...
✅ otelcol-contrib stopped.
```

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