inlet: decode MPLS labels

They are stored in an array and there are some aliases to get 1st, 2nd
and third label. Support for sFlow would need a test to ensure it works
as expected.

Fix #960

console/query/column_test.go

@@ -92,6 +92,12 @@ func TestQueryColumnSQLSelect(t *testing.T) {
 		}, {
 			Input:    schema.ColumnDstMAC,
 			Expected: `MACNumToString(DstMAC)`,
+		}, {
+			Input:    schema.ColumnMPLSLabels,
+			Expected: `arrayStringConcat(MPLSLabels, ' ')`,
+		}, {
+			Input:    schema.ColumnMPLS3rdLabel,
+			Expected: `toString(MPLS3rdLabel)`,
 		}, {
 			Input: schema.ColumnTCPFlags,
 			// Can be tested with "WITH 16 AS TCPFlags SELECT ..."