tsweb/varz: add support for ShardedInt metrics

Fixes tailscale/corp#33236

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

tsweb/varz/varz.go

@@ -25,6 +25,7 @@ import (
 
 	"golang.org/x/exp/constraints"
 	"tailscale.com/metrics"
+	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 	"tailscale.com/version"
 )
@@ -136,6 +137,9 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 	case *expvar.Int:
 		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, cmp.Or(typ, "counter"), name, v.Value())
 		return
+	case *syncs.ShardedInt:
+		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, cmp.Or(typ, "counter"), name, v.Value())
+		return
 	case *expvar.Float:
 		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, cmp.Or(typ, "gauge"), name, v.Value())
 		return