Diagnostics with ActivitySource

[alexeyzimarev]

Use the ActivitySource API to enable distributed tracing.

It's not a strict requirement as HttpClient is already instrumented, but it would be nice to see how much time it takes to make a call, and what time RestSharp takes for pre- and post-processing.

[hanhanyao]

Any update of adding OpenTelemetry integration

[alexeyzimarev]

Not yet; the default HTTP client instrumentation works just fine, except it doesn't measure the overhead (serialisation, etc.).