DISCUSSION PROPOSAL: Polly eventing and metrics architecture

ℹ️ This documentation describes the previous Polly v7 API. If you are using the new v8 API, please refer to pollydocs.org.

The original document here has been deprecated. Newer techniques such as System.Diagnostics.DiagnosticSource and evolutions in the ecosystem such as OpenTracing/OpenTelemetry present newer opportunities.