KObj: optimize string concatenation

Using a strings.Builder reduces the number of allocations:

name                               old time/op    new time/op    delta
KlogOutput/KObjSlice_okay-36         15.2µs ± 5%    14.8µs ± 4%    ~     (p=0.151 n=5+5)
KlogOutput/KObjSlice_nil_entry-36    14.4µs ± 5%    13.6µs ± 3%  -5.25%  (p=0.032 n=5+5)
KlogOutput/KObj-36                   13.5µs ± 8%    13.5µs ± 6%    ~     (p=0.841 n=5+5)
KlogOutput/KObjSlice_ints-36         15.3µs ± 5%    15.2µs ± 4%    ~     (p=0.841 n=5+5)
KlogOutput/KObjSlice_nil_arg-36      12.8µs ± 2%    12.8µs ± 6%    ~     (p=0.841 n=5+5)
KlogOutput/KObjSlice_int_arg-36      14.1µs ± 4%    13.8µs ± 3%    ~     (p=0.310 n=5+5)
KlogOutput/KObjs-36                  14.1µs ± 8%    13.6µs ± 8%    ~     (p=0.690 n=5+5)

name                               old alloc/op   new alloc/op   delta
KlogOutput/KObjSlice_okay-36         2.89kB ± 0%    2.82kB ± 0%  -2.23%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_entry-36    2.65kB ± 0%    2.62kB ± 0%    ~     (p=0.079 n=4+5)
KlogOutput/KObj-36                   2.50kB ± 0%    2.47kB ± 0%  -1.30%  (p=0.000 n=4+5)
KlogOutput/KObjSlice_ints-36         2.90kB ± 0%    2.90kB ± 0%    ~     (p=1.000 n=5+5)
KlogOutput/KObjSlice_nil_arg-36      2.41kB ± 0%    2.41kB ± 0%    ~     (all equal)
KlogOutput/KObjSlice_int_arg-36      2.67kB ± 0%    2.67kB ± 0%    ~     (all equal)
KlogOutput/KObjs-36                  2.72kB ± 0%    2.65kB ± 0%  -2.38%  (p=0.008 n=5+5)

name                               old allocs/op  new allocs/op  delta
KlogOutput/KObjSlice_okay-36           46.0 ± 0%      42.0 ± 0%  -8.70%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_entry-36      40.0 ± 0%      38.0 ± 0%  -5.00%  (p=0.008 n=5+5)
KlogOutput/KObj-36                     36.0 ± 0%      34.0 ± 0%  -5.56%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_ints-36           39.0 ± 0%      39.0 ± 0%    ~     (all equal)
KlogOutput/KObjSlice_nil_arg-36        35.0 ± 0%      35.0 ± 0%    ~     (all equal)
KlogOutput/KObjSlice_int_arg-36        37.0 ± 0%      37.0 ± 0%    ~     (all equal)
KlogOutput/KObjs-36                    42.0 ± 0%      38.0 ± 0%  -9.52%  (p=0.008 n=5+5)

k8s_references.go

@@ -19,6 +19,7 @@ package klog
 import (
 	"fmt"
 	"reflect"
+	"strings"
 
 	"github.com/go-logr/logr"
 )
@@ -31,7 +32,12 @@ type ObjectRef struct {
 
 func (ref ObjectRef) String() string {
 	if ref.Namespace != "" {
-		return fmt.Sprintf("%s/%s", ref.Namespace, ref.Name)
+		var builder strings.Builder
+		builder.Grow(len(ref.Namespace) + len(ref.Name) + 1)
+		builder.WriteString(ref.Namespace)
+		builder.WriteRune('/')
+		builder.WriteString(ref.Name)
+		return builder.String()
 	}
 	return ref.Name
 }