Update interceptors example to use new GetValue method (#1624)

Author: JamesNK

examples/Interceptor/Server/ServerLoggerInterceptor.cs

@@ -94,8 +94,8 @@ private void LogCall<TRequest, TResponse>(MethodType methodType, ServerCallConte
 
             void WriteMetadata(Metadata headers, string key)
             {
-                var headerValue = headers.SingleOrDefault(h => h.Key == key)?.Value;
-                _logger.LogWarning($"{key}: {headerValue ?? "(unknown)"}");
+                var headerValue = headers.GetValue(key) ?? "(unknown)";
+                _logger.LogWarning($"{key}: {headerValue}");
             }
         }
     }