storagenode/piecestore/usedserials: add monkit metric for serials that are randomly deleted

This will give storagenode operators a better idea of whether the memory
allocated to the usedserials store is sufficient.

Change-Id: I5c30f2e39473a573f43409511ad9e2e32680479c

monkit.lock

@@ -98,3 +98,4 @@ storj.io/storj/satellite/satellitedb."unknown_audit_reputation_beta" FloatVal
 storj.io/storj/storage/filestore."open_file_in_trash" Meter
 storj.io/storj/storagenode/contact."satellite_contact_request" Meter
 storj.io/storj/storagenode/gracefulexit."satellite_gracefulexit_request" Meter
+storj.io/storj/storagenode/piecestore/usedserials."delete_random_serial" Meter

storagenode/piecestore/usedserials/table.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/spacemonkeygo/monkit/v3"
 	"github.com/zeebo/errs"
 
 	"storj.io/common/memory"
@@ -21,6 +22,8 @@ var (
 	ErrSerials = errs.Class("used serials store error")
 	// ErrSerialAlreadyExists defines an error class for duplicate usedserials.
 	ErrSerialAlreadyExists = errs.Class("used serial already exists in store")
+
+	mon = monkit.Package()
 )
 
 const (
@@ -198,6 +201,7 @@ func (table *Table) Count() int {
 // deleteRandomSerial deletes a random item.
 // It expects the mutex to be locked before being called.
 func (table *Table) deleteRandomSerial() error {
+	mon.Meter("delete_random_serial").Mark(1) //locked
 	for _, satMap := range table.serials {
 		for expirationHour, serialList := range satMap {
 			if len(serialList.partialSerials) > 0 {