Summary

The .NET SDK currently sets no Qdrant-branded User-Agent. The only token on the wire is the one Grpc.Net.Client injects, e.g.:

grpc-dotnet/2.71.0 (.NET 8.0.4; CLR 8.0.4; net8.0; linux; x64)

Because of that, server-side tooling cannot attribute traffic to the .NET client. Concretely, the Qdrant Cloud auth sidecars expose a client_requests_total{client,version} metric resolved from the User-Agent, and .NET requests land in the other bucket instead of dotnet.

This PR makes the SDK advertise a qdrant-dotnet/<version> token.

Approach

Grpc.Net.Client does not allow replacing the User-Agent via gRPC metadata (it only appends), so the token is added at the HTTP layer with a small DelegatingHandler (UserAgentHandler) wired into QdrantChannel.ForAddress. The resulting header looks like:

grpc-dotnet/2.71.0 (...) qdrant-dotnet/1.18.0

• Version is resolved from the assembly informational version (stamped by MinVer at build time), with build metadata (+<sha>) stripped.
• The handler adds rather than clears, so the gRPC/runtime information is preserved, and it de-dupes so a re-sent request doesn't accumulate duplicate tokens.
• Wired for net6.0/netstandard2.0. On .NET Framework (net462), where the channel's HTTP handler must be configured manually (WinHttpHandler, per the README), the original behavior is preserved to avoid disturbing the finicky HTTP/2 setup.

Tests

Added UserAgentHandlerTests (pure unit tests, no Qdrant server needed):

• adds the qdrant-dotnet/<version> token
• preserves an existing grpc-dotnet token
• does not duplicate the token when a request is re-sent (retry)

Notes

• Could not compile locally (no .NET SDK in this environment); relying on CI (net8.0 test run + net462/net6.0/netstandard2.0 build) to validate.

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