[#3] [add] impl

(cherry picked from commit ac1878d)

Author: i4004

src/Etcd.Microsoft.Extensions.Configuration/Client/EtcdKeyValueClient.cs

@@ -149,7 +149,14 @@ private void Authenticate(ICredentials credentials)
 		_userName = credentials.UserName;
 	}
 
-	private IEnumerable<Mvccpb.KeyValue> GetKeys(string prefixKey) => _client.GetRange(prefixKey, GetMetadata()).Kvs;
+	private IEnumerable<Mvccpb.KeyValue> GetKeys(string prefixKey)
+	{
+		// Fix to correct read all keys
+		if (prefixKey == "\0")
+			prefixKey = "";
+
+		return _client.GetRange(prefixKey, GetMetadata()).Kvs;
+	}
 
 	private IEnumerable<string> GetRoles() =>
 		_client.UserGet(new AuthUserGetRequest