enhancement(multitenant): Track rounding error in billing calculation

Billing takes an integer number of seconds, so keep track of the
amount lost to rounding when the publish interval is not an integer.

app/multitenant/billing_emitter.go

@@ -4,6 +4,7 @@ import (
 	"crypto/sha256"
 	"encoding/base64"
 	"flag"
+	"math"
 	"strings"
 	"sync"
 	"time"
@@ -37,6 +38,7 @@ type BillingEmitter struct {
 
 	sync.Mutex
 	intervalCache map[string]time.Duration
+	rounding      map[string]float64
 }
 
 // NewBillingEmitter changes a new billing emitter which emits billing events
@@ -46,6 +48,7 @@ func NewBillingEmitter(upstream app.Collector, billingClient *billing.Client, cf
 		billing:              billingClient,
 		BillingEmitterConfig: cfg,
 		intervalCache:        make(map[string]time.Duration),
+		rounding:             make(map[string]float64),
 	}, nil
 }
 
@@ -84,9 +87,14 @@ func (e *BillingEmitter) Add(ctx context.Context, rep report.Report, buf []byte)
 		weaveNetCount = 1
 	}
 
+	// Billing takes an integer number of seconds, so keep track of the amount lost to rounding
+	nodeSeconds := interval.Seconds()*float64(len(rep.Host.Nodes)) + e.rounding[userID]
+	rounding := nodeSeconds - math.Floor(nodeSeconds)
+	e.rounding[userID] = rounding
+
 	amounts := billing.Amounts{
 		billing.ContainerSeconds: int64(interval/time.Second) * int64(len(rep.Container.Nodes)),
-		billing.NodeSeconds:      int64(interval/time.Second) * int64(len(rep.Host.Nodes)),
+		billing.NodeSeconds:      int64(nodeSeconds),
 		billing.WeaveNetSeconds:  int64(interval/time.Second) * int64(weaveNetCount),
 	}
 	metadata := map[string]string{