attack: Force monotonic timestamp marshalling (tsenart#377)

* attack: Force monotonic timestamp marshalling

This commit forces result timestamps to be monotonically increasing.

This is necessary because the built-in monotonic time reading of the standard
library time package is discarded when marshalling a time.Time instance.

This can lead to non-monotonic timestamps in vegeta results of a single
attack.

Although an explicit decision as stated in the time package documentation,
this assumption seems surprising to me.

From: https://golang.org/pkg/time/#hdr-Monotonic_Clocks

> Because the monotonic clock reading has no meaning outside the current
> process, the serialized forms generated by t.GobEncode, t.MarshalBinary,
> t.MarshalJSON, and t.MarshalText omit the monotonic clock reading,
> and t.Format provides no format for it. Similarly, the constructors
> time.Date, time.Parse, time.ParseInLocation, and time.Unix, as well as
> the unmarshalers t.GobDecode, t.UnmarshalBinary. t.UnmarshalJSON, and
> t.UnmarshalText always create times with no monotonic clock reading.

* Add github.com/gavv/monotime to Gopkg.*

* fixup! Remove redundant monotime dependency

lib/attack.go

@@ -24,6 +24,7 @@ type Attacker struct {
 	redirects int
 	seqmu     sync.Mutex
 	seq       uint64
+	began     time.Time
 }
 
 const (
@@ -59,6 +60,7 @@ func NewAttacker(opts ...func(*Attacker)) *Attacker {
 		stopch:  make(chan struct{}),
 		workers: DefaultWorkers,
 		maxBody: DefaultMaxBody,
+		began:   time.Now(),
 	}
 
 	a.dialer = &net.Dialer{
@@ -69,8 +71,8 @@ func NewAttacker(opts ...func(*Attacker)) *Attacker {
 
 	a.client = http.Client{
 		Transport: &http.Transport{
-			Proxy: http.ProxyFromEnvironment,
-			Dial:  a.dialer.Dial,
+			Proxy:                 http.ProxyFromEnvironment,
+			Dial:                  a.dialer.Dial,
 			ResponseHeaderTimeout: DefaultTimeout,
 			TLSClientConfig:       DefaultTLSConfig,
 			TLSHandshakeTimeout:   10 * time.Second,
@@ -291,7 +293,7 @@ func (a *Attacker) hit(tr Targeter, name string) *Result {
 	}()
 
 	a.seqmu.Lock()
-	res.Timestamp = time.Now()
+	res.Timestamp = a.began.Add(time.Since(a.began))
 	res.Seq = a.seq
 	a.seq++
 	a.seqmu.Unlock()