Added link to the HttpTelemetry Event Source

[danespinosa]

Summary

Adds link to the Event Source for System.Net.Http, System.Net.Security, System.Net.NameResolution and System.Net.Sockets to keep consistency across the different Event Sources.

Fixes #Issue_Number (if available)

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docs/core/diagnostics/available-counters.md	Well-known EventCounters in .NET