Introduce ResilienceTelemetryFactory

[martintmk]

The issue or feature being addressed

#1066

Details on the issue fix or feature implementation

This API exposes simple ResilienceTelemetry API that the ResilienceStrategy authors can use to report important events in a unified way. For example:

ResilienceTelemetry telemetry = ...;

telemetry.Report("OnTimeout", resilienceContext);
 
telemetry.Report("OnRetry", failedResult, resilienceContext);
 
telemetry.ReportException("OnRetry", failedException, resilienceContext);

The consumers of ResilienceStrategy can provide their own implementation of ResilienceTelemetryFactory to provide additional telemetry with each event (extracted from ResilienceContext or from reported result or exception).

By default, we use NullResilienceTelemetryFactory that does nothing. At later stages we might consider introducing Polly.Telemetry package that implements ResilienceTelemetryFactory and does logging using ILoggerFactory and metering with the new Meter API.

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build