public static long getTimestamp(long timeBucket) {
        if (isSecondBucket(timeBucket)) {
            return getTimestamp(timeBucket, DownSampling.Second);
        } else if (isMinuteBucket(timeBucket)) {
            return getTimestamp(timeBucket, DownSampling.Minute);
        } else if (isHourBucket(timeBucket)) {
            return getTimestamp(timeBucket, DownSampling.Hour);
        } else if (isDayBucket(timeBucket)) {
            return getTimestamp(timeBucket, DownSampling.Day);
        } else {
            throw new UnsupportedOperationException("Unknown downsampling value.");
        }
    }
Convert TimeBucket to Timestamp in millisecond.

@param timeBucket long
@return timestamp in millisecond unit
test/e2e-v2/java-test-service/e2e-mock-baseline-server/src/main/java/org/apache/skywalking/e2e/TimeBucket.java