gRPC Interceptor to check for an X-Obsolete header

ObsoleteLoggingInterceptor.cs

    internal class ObsoleteLoggingInterceptor : Interceptor
    {
        private readonly ILogger<ObsoleteLoggingInterceptor> _logger;

        public ObsoleteLoggingInterceptor(ILogger<ObsoleteLoggingInterceptor> logger)
        {
            _logger = logger;
        }

        public override AsyncUnaryCall<TResponse> AsyncUnaryCall<TRequest, TResponse>(TRequest request,
            ClientInterceptorContext<TRequest, TResponse> context,
            AsyncUnaryCallContinuation<TRequest, TResponse> continuation)
        {
            var call = continuation(request, context);
            return new AsyncUnaryCall<TResponse>(call.ResponseAsync,
                CheckObsoleteHeaderAsync(context.Method.Name, call.ResponseHeadersAsync),
                call.GetStatus, call.GetTrailers, call.Dispose);
        }
        
        // Override other methods as necessary

        private async Task<Metadata> CheckObsoleteHeaderAsync(string methodName, Task<Metadata> responseHeadersAsync)
        {
            var metadata = await responseHeadersAsync;

            var obsolete = metadata.FirstOrDefault(e => string.Equals(e.Key, "x-obsolete", StringComparison.OrdinalIgnoreCase));
            if (!(obsolete is null))
            {
                _logger.LogWarning("{MethodName} is obsolete: {Message}", methodName, obsolete.Value);
            }

            return metadata;
        }
    }