Improve Telemetry Lesson: Replace API placeholder and add cache metrics documentation

[Copilot]

This PR addresses two key issues in the telemetry lesson that were making it difficult for workshop participants to follow along:

1. Replaced API Code Placeholder

The README.md contained a placeholder comment // ... API call logic ... instead of the actual working code implementation. This was error-prone and required participants to guess or look elsewhere for the complete code.

Before:

try 
{
    // ... API call logic ...
    stopwatch.Stop();
    // ... rest of implementation
}

After:

try 
{
    // Create an exception every 5 calls to simulate an error for testing
    if (forecastCount % 5 == 0)
    {
        throw new Exception("Random exception thrown by NwsManager.GetForecastAsync");
    }

    var zoneIdSegment = HttpUtility.UrlEncode(zoneId);
    var forecasts = await httpClient.GetFromJsonAsync<ForecastResponse>($"zones/forecast/{zoneIdSegment}/forecast", options);
    stopwatch.Stop();
    
    // Record the request duration
    NwsManagerDiagnostics.forecastRequestDuration.Record(stopwatch.Elapsed.TotalSeconds);
    activity?.SetTag("request.success", true);

    var result = forecasts
        ?.Properties
        ?.Periods
        ?.Select(p => (Forecast)p)
        .ToArray() ?? [];

    logger.LogInformation(
        "📊 Retrieved forecast for zone {ZoneId} in {Duration:N0}ms with {PeriodCount} periods",
        zoneId,
        stopwatch.Elapsed.TotalMilliseconds,
        result.Length
    );

    return result;
}

2. Added Complete Cache Metrics Documentation

The lesson defined cache_hits_total and cache_misses_total metrics but didn't properly document or implement their usage. Added a comprehensive "Implementing Cache Metrics" section that:

• Shows how to properly track both cache hits and misses using cache.TryGetValue() pattern
• Explains the difference between cache hit and miss scenarios
• Demonstrates activity tracing tags for distributed tracing
• Provides structured logging examples for cache performance monitoring

3. Enhanced Lesson Code Implementation

Updated the GetZonesAsync method in the lesson code to properly implement cache hit tracking:

// Check if data exists in cache first
if (cache.TryGetValue("zones", out Zone[]? cachedZones))
{
    // Record cache hit
    NwsManagerDiagnostics.cacheHitCounter.Add(1);
    activity?.SetTag("cache.hit", true);
    
    logger.LogInformation("📈 Retrieved {ZoneCount} zones from cache (cache hit)", cachedZones?.Length ?? 0);
    return cachedZones;
}

These changes make the telemetry lesson more complete and easier to follow, providing participants with working code they can copy-paste without errors.

Fixes #185.

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[Copilot]

Pull Request Overview

This PR enhances the telemetry lesson by replacing incomplete placeholder code with fully functional implementations and adding comprehensive cache metrics documentation. The changes make the workshop more complete and easier for participants to follow.

Key changes:

• Replaced API placeholder comment with complete working code implementation including error simulation
• Added comprehensive cache metrics implementation section with proper hit/miss tracking
• Enhanced the GetZonesAsync method with activity tracing and structured logging

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
workshop/Lesson-06-Telemetry/code/Api/Data/NwsManager.cs	Implements cache hit tracking with activity tracing and improved logging
workshop/Lesson-06-Telemetry/README.md	Replaces API placeholder with complete implementation and adds cache metrics documentation