Infrastructure for telemetry

[martintmk]

We want to expose an infrastructure for telemetry. This way, individual resilience strategies can report important events in a unified way. The Polly consumers can hook to these events and build monitoring and dashboard on top.

We can consider something like:

public abstract class ResilienceTelemetry
{
    void Report(string eventName, ResilienceContext context);

    void Report<T>(string eventName, T result, ResilienceContext context);

    void Report(string eventName, Exception exception, ResilienceContext context);
}

public abstract class ResilienceTelemetryFactory
{
    ResilienceTelemetry Create(ResilienceTelemetryContext context);
}

public class ResilienceStrategyBuilderContext
{
    // new property
    public ResilienceTelemetry Telemetry { get; }
}

public class ResilienceStrategyBuilderOptions
{
    // new property
    public ResilienceTelemetryFactory TelemetryFactory { get; set; } = NullResilienceTelemetryFactory.Instance;
}

Customer can provide their own implementation of ResilienceTelemetryFactory. We can consider having built-in ResilienceTelemetryFactory based on .NET Meter and ILoggerFactory.

The ResilienceTelemetryFactory.Create is called for each resilience strategy individually and with different context.

[martintmk]

The API was simplified in #1112 and it is now using DiagnosticSource internally.