vtorc: add tablets watched stats

Signed-off-by: Tim Vaillancourt <[email protected]>

go/vt/vtorc/logic/tablet_discovery.go

@@ -31,6 +31,7 @@ import (
 	"google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/proto"
 
+	"vitess.io/vitess/go/stats"
 	"vitess.io/vitess/go/vt/external/golib/sqlutils"
 	"vitess.io/vitess/go/vt/key"
 	"vitess.io/vitess/go/vt/log"
@@ -54,6 +55,13 @@ var (
 	// This is populated by parsing `--clusters_to_watch` flag.
 	shardsToWatch map[string][]*topodatapb.KeyRange
 
+	// tabletsWatchedByCell is a map of the number of tablets watched by cell.
+	tabletsWatchedByCell map[string]int64
+	statsTabletsWatched  = stats.NewGaugesFuncWithMultiLabels("TabletsWatched", "Number of tablets watched by cell",
+		[]string{"cell"},
+		func() map[string]int64 { return tabletsWatchedByCell },
+	)
+
 	// ErrNoPrimaryTablet is a fixed error message.
 	ErrNoPrimaryTablet = errors.New("no primary tablet found")
 )
@@ -168,6 +176,7 @@ func getAllTablets(ctx context.Context, cells []string) (tabletsByCell map[strin
 				failedCells = append(failedCells, cell)
 			} else {
 				tabletsByCell[cell] = tablets
+				tabletsWatchedByCell[cell] = int64(len(tablets))
 			}
 			return nil
 		})