feat(metrics): add performance timing helpers

[CalvinAllen]

Summary

Add simple, ergonomic helpers for timing operations and recording durations to histograms. Reduces boilerplate for the common pattern of measuring how long something takes.

Proposed API

// Simple timing with auto-record to histogram
var timing = VsixTelemetry.StartTiming();
// ... do work ...
timing.Record("MyOperation"); // Records to vs.extension.operation.duration

// Timing with custom histogram
var timing = VsixTelemetry.StartTiming();
// ... do work ...
timing.RecordTo(myCustomHistogram, tags);

// One-liner for sync operations
var result = VsixTelemetry.Timed("LoadData", () => LoadDataFromDatabase());

// Async version
var result = await VsixTelemetry.TimedAsync("LoadData", async () => 
    await LoadDataFromDatabaseAsync());

// With automatic span creation
var result = VsixTelemetry.TimedWithSpan("LoadData", () => 
{
    // Creates span AND records duration
    return LoadDataFromDatabase();
});

Timing Class

public class OperationTiming
{
    public TimeSpan Elapsed { get; }
    public void Record(string operationName, params KeyValuePair<string, object>[] tags);
    public void RecordTo<T>(Histogram<T> histogram, params KeyValuePair<string, object>[] tags);
    public void AddTag(string key, object value);
}

Benefits

• Less boilerplate than manual Stopwatch usage
• Consistent metric naming and tagging
• Easy to add timing to existing code
• Combines well with span creation