[API Proposal]: Allow adding links after activity creation

[lmolkova]

Background and motivation

OpenTelemetry recently added and stabilized adding links after span creation.

See https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#link for the details.

This is useful:

• for messaging/event processing scenarios - when messages are pulled from the messaging system, the trace-context they carry is not known until messages are received
• associating phases of establishing connections and HTTP requests - see Consider adding activities for potentially long running network operations #93832 or Add client semantic conventions for socket connections  open-telemetry/semantic-conventions#756 for the context

API Proposal

Add Activity.AddLink(ActivityLink link) API

API Usage

using var activity = Source.StartActivity("Receive");

var messages = await ReceiveInternal(maxBtchSize);
foreach (var ctx in ExtractTraceContext(messages))
{
    activity.AddLink(new ActivityLink(ctx)); // this is new
}

Alternative Designs

No response

Risks

No response

[dotnet-policy-service]

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[tarekgh]

Duplicate of #97680