Finalize experimental connection Activities

[antonfirsov]

#103922 introduced experimental connection tracing, adding 4 optional activities to enable tracking HTTP connection setup, DNS lookup, socket connect, and TLS handshake. The feature has been marked experimental because of problems with span link visualization in existing monitoring tools (open-telemetry/opentelemetry-specification#4130). These problems can be addressed by (1) introducing new OTel semantic conventions to mark the links that can result in a high number of backlings so they can be visualized differently, (2) implementing the new semconv in our instrumentation, and (3) working with monitoring tool authors so they can tune their visualization logic.

During the finalization work we have an opportunity to adjust the design and the implementation meaning that we can revisit the design decisions noted in #103922 (comment), and formalized in open-telemetry/semantic-conventions#1192. For example, we should consider linking failed connection setup spans to the (failed) requests that initiated the connection injection as suggested in dotnet/docs#43872 (comment).

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