[Feature]: OpenTelemetry / Diagnostics Integration

[eneshoxha]

Title: Add OpenTelemetry tracing behaviors via System.Diagnostics.Activity

Labels: enhancement, mediator, new-package

Body:

Problem

The built-in logging behaviors use ILogger with Stopwatch for timing. Modern .NET applications expect distributed tracing via System.Diagnostics.Activity and OpenTelemetry. Without this, Cortex.Mediator operations are invisible in distributed traces.

Proposed Solution

Create a Cortex.Mediator.OpenTelemetry package:

• Define an ActivitySource named Cortex.Mediator
• TracingCommandBehavior<TCommand, TResult> -- starts an Activity span for command execution
• TracingQueryBehavior<TQuery, TResult> -- same for queries
• TracingNotificationBehavior<TNotification> -- same for notifications
• Tags: mediator.message.type (command/query/notification), mediator.message.name, mediator.handler.name, mediator.result.status (success/failure)
• Set Activity status to Error on exceptions

Registration:

options.AddOpenTelemetryBehaviors();

// In OpenTelemetry configuration:
builder.Services.AddOpenTelemetry()
    .WithTracing(tracing => tracing.AddSource("Cortex.Mediator"));