Fix scrape timestamps to reduce sample time jitter.

We're currently timestamping samples with the time at the end of a scrape
iteration. It makes more sense to use a timestamp from the beginning of the
scrape for two reasons:

a) this time is more relevant to the scraped values than the time at the
   end of the HTTP-GET + JSON decoding work.
b) it reduces sample timestamp jitter if we measure at the beginning, and
   not at the completion of a scrape.

retrieval/format/processor.go

@@ -16,6 +16,7 @@ package format
 import (
 	"github.com/prometheus/prometheus/model"
 	"io"
+	"time"
 )
 
 // Processor is responsible for decoding the actual message responses from
@@ -23,5 +24,5 @@ import (
 // to the results channel.
 type Processor interface {
 	// Process performs the work on the input and closes the incoming stream.
-	Process(stream io.ReadCloser, baseLabels model.LabelSet, results chan Result) (err error)
+	Process(stream io.ReadCloser, timestamp time.Time, baseLabels model.LabelSet, results chan Result) (err error)
 }

retrieval/format/processor0_0_1.go

@@ -20,6 +20,7 @@ import (
 	"github.com/prometheus/prometheus/utility"
 	"io"
 	"io/ioutil"
+	"time"
 )
 
 const (
@@ -57,7 +58,7 @@ type entity001 []struct {
 	} `json:"metric"`
 }
 
-func (p *processor001) Process(stream io.ReadCloser, baseLabels model.LabelSet, results chan Result) (err error) {
+func (p *processor001) Process(stream io.ReadCloser, timestamp time.Time, baseLabels model.LabelSet, results chan Result) (err error) {
 	// TODO(matt): Replace with plain-jane JSON unmarshalling.
 	defer stream.Close()
 
@@ -73,8 +74,6 @@ func (p *processor001) Process(stream io.ReadCloser, baseLabels model.LabelSet,
 		return
 	}
 
-	now := p.time.Now()
-
 	// TODO(matt): This outer loop is a great basis for parallelization.
 	for _, entity := range entities {
 		for _, value := range entity.Metric.Value {
@@ -101,7 +100,7 @@ func (p *processor001) Process(stream io.ReadCloser, baseLabels model.LabelSet,
 
 				sample := model.Sample{
 					Metric:    metric,
-					Timestamp: now,
+					Timestamp: timestamp,
 					Value:     model.SampleValue(sampleValue),
 				}
 
@@ -136,7 +135,7 @@ func (p *processor001) Process(stream io.ReadCloser, baseLabels model.LabelSet,
 
 					sample := model.Sample{
 						Metric:    childMetric,
-						Timestamp: now,
+						Timestamp: timestamp,
 						Value:     model.SampleValue(individualValue),
 					}
 

retrieval/format/processor0_0_1_test.go

@@ -21,6 +21,7 @@ import (
 	"io/ioutil"
 	"strings"
 	"testing"
+	"time"
 )
 
 func testProcessor001Process(t test.Tester) {
@@ -172,7 +173,7 @@ func testProcessor001Process(t test.Tester) {
 
 		reader := strings.NewReader(scenario.in)
 
-		err := Processor001.Process(ioutil.NopCloser(reader), model.LabelSet{}, inputChannel)
+		err := Processor001.Process(ioutil.NopCloser(reader), time.Now(), model.LabelSet{}, inputChannel)
 		if !test.ErrorEqual(scenario.err, err) {
 			t.Errorf("%d. expected err of %s, got %s", i, scenario.err, err)
 			continue

retrieval/target.go

@@ -162,6 +162,8 @@ func (t *target) Scrape(earliest time.Time, results chan format.Result) (err err
 			done <- true
 		}()
 
+		now := time.Now()
+
 		var resp *http.Response // Don't shadow "err" from the enclosing function.
 		resp, err = http.Get(t.Address())
 		if err != nil {
@@ -182,7 +184,7 @@ func (t *target) Scrape(earliest time.Time, results chan format.Result) (err err
 			baseLabels[baseLabel] = baseValue
 		}
 
-		err = processor.Process(resp.Body, baseLabels, results)
+		err = processor.Process(resp.Body, now, baseLabels, results)
 		if err != nil {
 			return
 		}