feat(summary): add count in percentiles

To measure the quality of the derived metric, it is necessary to also
store the count of the samples from which the percentile calculation is
being done. This commit aims to add this feature in cadvisor.

Author: sambhav-jain-16

info/v2/container.go

@@ -200,6 +200,8 @@ type Percentiles struct {
 	Ninety uint64 `json:"ninety"`
 	// 95th percentile over the collected sample.
 	NinetyFive uint64 `json:"ninetyfive"`
+	// Number of samples used to calculate these percentiles.
+	Count uint64 `json:"count"`
 }
 
 type Usage struct {

summary/percentiles.go

@@ -109,6 +109,7 @@ func (r *resource) AddSample(val uint64) {
 		Fifty:      val,
 		Ninety:     val,
 		NinetyFive: val,
+		Count:      1,
 	}
 	r.Add(sample)
 }
@@ -121,6 +122,8 @@ func (r *resource) GetAllPercentiles() info.Percentiles {
 	p.Fifty = r.samples.GetPercentile(0.5)
 	p.Ninety = r.samples.GetPercentile(0.9)
 	p.NinetyFive = r.samples.GetPercentile(0.95)
+	// len(samples) is equal to count stored in mean.
+	p.Count = r.mean.count
 	p.Present = true
 	return p
 }

summary/percentiles_test.go

@@ -84,6 +84,8 @@ func TestAggregates(t *testing.T) {
 		Fifty:      1000,
 		Ninety:     1000,
 		NinetyFive: 1000,
+		// Since cpu is calculated between samples, we lose 1 sample.
+		Count: N - 2,
 	}
 	if usage.Cpu != cpuExpected {
 		t.Errorf("cpu stats are %+v. Expected %+v", usage.Cpu, cpuExpected)
@@ -95,6 +97,7 @@ func TestAggregates(t *testing.T) {
 		Fifty:      50 * 1024,
 		Ninety:     90 * 1024,
 		NinetyFive: 95 * 1024,
+		Count:      N - 1,
 	}
 	if usage.Memory != memExpected {
 		t.Errorf("memory stats are mean %+v. Expected %+v", usage.Memory, memExpected)
@@ -133,6 +136,8 @@ func TestSamplesCloseInTimeIgnored(t *testing.T) {
 		Fifty:      1000,
 		Ninety:     1000,
 		NinetyFive: 1000,
+		// Since cpu is calculated between samples, we lose 1 sample.
+		Count: N - 2,
 	}
 	if usage.Cpu != cpuExpected {
 		t.Errorf("cpu stats are %+v. Expected %+v", usage.Cpu, cpuExpected)
@@ -144,6 +149,7 @@ func TestSamplesCloseInTimeIgnored(t *testing.T) {
 		Fifty:      50 * 1024,
 		Ninety:     90 * 1024,
 		NinetyFive: 95 * 1024,
+		Count:      N - 1,
 	}
 	if usage.Memory != memExpected {
 		t.Errorf("memory stats are mean %+v. Expected %+v", usage.Memory, memExpected)
@@ -184,6 +190,8 @@ func TestDerivedStats(t *testing.T) {
 		Fifty:      50 * Nanosecond,
 		Ninety:     90 * Nanosecond,
 		NinetyFive: 95 * Nanosecond,
+		// GetDerivedPercentiles calculates directly from samples, hence we don't lose any samples.
+		Count: N - 1,
 	}
 	if usage.Cpu != cpuExpected {
 		t.Errorf("cpu stats are %+v. Expected %+v", usage.Cpu, cpuExpected)
@@ -195,6 +203,7 @@ func TestDerivedStats(t *testing.T) {
 		Fifty:      50 * 1024,
 		Ninety:     90 * 1024,
 		NinetyFive: 95 * 1024,
+		Count:      N - 1,
 	}
 	if usage.Memory != memExpected {
 		t.Errorf("memory stats are mean %+v. Expected %+v", usage.Memory, memExpected)