otelzap: Add Benchmarks (#5784)

Part of #5191

Pre-work #5279

```
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/contrib/bridges/otelzap
cpu: 12th Gen Intel(R) Core(TM) i5-1245U
BenchmarkCoreWrite/10_fields-12         	 1000000	      1116 ns/op	     978 B/op	      13 allocs/op
BenchmarkCoreWrite/20_fields-12         	  407290	      2615 ns/op	    2180 B/op	      22 allocs/op
BenchmarkCoreWrite/Namespace-12         	  725268	      1671 ns/op	    1760 B/op	      16 allocs/op
BenchmarkCoreWrite/With10_fields-12     	 4964038	       260.2 ns/op	     208 B/op	       1 allocs/op
BenchmarkCoreWrite/With20_fields-12     	 2004085	       628.7 ns/op	     640 B/op	       1 allocs/op
BenchmarkCoreWrite/WithNamespace-12     	100000000	        16.95 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	go.opentelemetry.io/contrib/bridges/otelzap	8.619s
```

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

bridges/otelzap/core.go

@@ -156,7 +156,6 @@ func (o *Core) Write(ent zapcore.Entry, fields []zapcore.Field) error {
 	r.SetSeverity(convertLevel(ent.Level))
 	r.SetSeverityText(ent.Level.String())
 
-	// TODO: Handle zap.Namespace.
 	// TODO: Handle ent.LoggerName.
 
 	r.AddAttributes(o.attr...)

bridges/otelzap/core_test.go

@@ -5,7 +5,9 @@ package otelzap
 
 import (
 	"context"
+	"fmt"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -22,6 +24,10 @@ var (
 	loggerName  = "name"
 	testKey     = "key"
 	testValue   = "value"
+	testEntry   = zapcore.Entry{
+		Level:   zap.InfoLevel,
+		Message: testMessage,
+	}
 )
 
 func TestCore(t *testing.T) {
@@ -178,3 +184,99 @@ func TestConvertLevel(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkCoreWrite(b *testing.B) {
+	benchmarks := []struct {
+		name   string
+		fields []zapcore.Field
+	}{
+		{
+			name: "10 fields",
+			fields: []zapcore.Field{
+				zap.Int16("a", 1),
+				zap.String("k", "a"),
+				zap.Bool("k", true),
+				zap.Time("k", time.Unix(1000, 1000)),
+				zap.Binary("k", []byte{1, 2}),
+				zap.ByteString("k", []byte{1, 2}),
+				zap.Object("k", loggable{true}),
+				zap.Array("k", loggable{true}),
+				zap.String("k", "a"),
+				zap.Ints("k", []int{1, 2}),
+			},
+		},
+		{
+			name: "20 fields",
+			fields: []zapcore.Field{
+				zap.Int16("a", 1),
+				zap.String("k", "a"),
+				zap.Bool("k", true),
+				zap.Time("k", time.Unix(1000, 1000)),
+				zap.Binary("k", []byte{1, 2}),
+				zap.ByteString("k", []byte{1, 2}),
+				zap.Object("k", loggable{true}),
+				zap.String("k", "a"),
+				zap.Array("k", loggable{true}),
+				zap.Ints("k", []int{1, 2}),
+				zap.Int16("a", 1),
+				zap.String("k", "a"),
+				zap.Bool("k", true),
+				zap.Time("k", time.Unix(1000, 1000)),
+				zap.Binary("k", []byte{1, 2}),
+				zap.ByteString("k", []byte{1, 2}),
+				zap.Object("k", loggable{true}),
+				zap.Array("k", loggable{true}),
+				zap.String("k", "a"),
+				zap.Ints("k", []int{1, 2}),
+			},
+		},
+		{ // Benchmark with nested namespace
+			name: "Namespace",
+			fields: []zapcore.Field{
+				zap.Namespace("a"),
+				zap.Int16("a", 1),
+				zap.String("k", "a"),
+				zap.Bool("k", true),
+				zap.Time("k", time.Unix(1000, 1000)),
+				zap.Binary("k", []byte{1, 2}),
+				zap.Namespace("b"),
+				zap.Binary("k", []byte{1, 2}),
+				zap.Object("k", loggable{true}),
+				zap.String("k", "a"),
+				zap.Array("k", loggable{true}),
+				zap.Ints("k", []int{1, 2}),
+			},
+		},
+	}
+
+	for _, bm := range benchmarks {
+		b.Run(bm.name, func(b *testing.B) {
+			zc := NewCore(loggerName)
+			b.ReportAllocs()
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					err := zc.Write(testEntry, bm.fields)
+					if err != nil {
+						b.Errorf("Unexpected error: %v", err)
+					}
+				}
+			})
+		})
+	}
+
+	for _, bm := range benchmarks {
+		b.Run(fmt.Sprint("With", bm.name), func(b *testing.B) {
+			zc := NewCore(loggerName)
+			zc1 := zc.With(bm.fields)
+			b.ReportAllocs()
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					err := zc1.Write(testEntry, []zapcore.Field{})
+					if err != nil {
+						b.Errorf("Unexpected error: %v", err)
+					}
+				}
+			})
+		})
+	}
+}