metrics: measure idle time during builds

This measures the amount of time spent idle during the build. This is
done by collecting the set of task times, determining which sections
contain gaps where no task is running, and aggregating that duration
into a metric.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

util/progress/metricwriter_test.go

@@ -0,0 +1,46 @@
+package progress
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCalculateIdleTime(t *testing.T) {
+	for _, tt := range []struct {
+		started   []int64
+		completed []int64
+		ms        int64
+	}{
+		{
+			started:   []int64{0, 1, 3},
+			completed: []int64{2, 10, 5},
+			ms:        0,
+		},
+		{
+			started:   []int64{0, 3},
+			completed: []int64{2, 5},
+			ms:        1,
+		},
+		{
+			started:   []int64{3, 0, 7},
+			completed: []int64{5, 2, 10},
+			ms:        3,
+		},
+	} {
+		started := unixMillis(tt.started...)
+		completed := unixMillis(tt.completed...)
+
+		actual := int64(calculateIdleTime(started, completed) / time.Millisecond)
+		assert.Equal(t, tt.ms, actual)
+	}
+}
+
+func unixMillis(ts ...int64) []time.Time {
+	times := make([]time.Time, len(ts))
+	for i, ms := range ts {
+		times[i] = time.UnixMilli(ms)
+	}
+	return times
+}