Preserve current activity for gRPC calls without diagnostics

[mtaghavi2005]

Problem

Fixes #2516

Grpc.Net.Client currently creates a fallback gRPC client Activity when Activity.Current is not null, even when gRPC diagnostics, logging, and ActivitySource listeners are not enabled.

That fallback activity was originally added to enable activity propagation. However, it replaces the caller's current logical activity during the send path. In that case, downstream propagation can use the hidden gRPC activity as the parent instead of the caller's activity, which can produce an incorrect parent-child span relationship.

Fix

Only create a gRPC client activity when it is actually needed for:

• diagnostic source events
• logging
• Grpc.Net.Client ActivitySource listeners

When none of those are enabled, the existing Activity.Current is preserved during send so lower-level HTTP/client instrumentation can propagate the caller's logical context.

Test

Added CurrentActivity_MakeCallWithoutDiagnostics_PreservedDuringSend to verify that Activity.Current is preserved during the send path when gRPC diagnostics are not enabled.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mtaghavi2005 / name: Mohammad Taghavi (544194b)