Fix benchmark artifact parsing.

* Added a extractor to extract raw logs from json format and then pipe
it into benchmark parser.
* Also added -alsologtostderr=false -logtostderr=false to reduce noisy logs.

test/integration/benchmark/extractlog/main.go

@@ -0,0 +1,61 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package main
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	err := extractRawLog(os.Stdin)
+	if err != nil {
+		panic(err)
+	}
+}
+
+// A json log entry contains keys such as "Time", "Action", "Package" and "Output".
+// We are only interested in "Output", which is the raw log.
+type jsonLog struct {
+	Output string `json:"output,omitempty"`
+}
+
+// jsonToRawLog converts a single line of json formatted log to raw log.
+// If there is an error, it returns the original input.
+func jsonToRawLog(line string) (string, error) {
+	var log jsonLog
+	if err := json.Unmarshal([]byte(line), &log); err != nil {
+		return line, err
+	}
+	return log.Output, nil
+}
+
+func extractRawLog(r io.Reader) error {
+	scan := bufio.NewScanner(r)
+	for scan.Scan() {
+		l, _ := jsonToRawLog(scan.Text())
+		// Print the raw log to stdout.
+		fmt.Println(l)
+	}
+	if err := scan.Err(); err != nil {
+		return err
+	}
+	return nil
+}