Improve metadata unmarshal performance

While profiling a application that uses goengine. The amount of memory used for Unmarshalling metadata was extreme.
By using easyjson we can remove the `map[string]interface{}` unmarshal and enjoy the speed and memory reduction is library offers.

Before this PR the add benchmark was:
```
BenchmarkJSONMetadata_UnmarshalJSON-4   	  300000	      4313 ns/op	    1208 B/op	      29 allocs/op
```
With the new and improved version it has become:
```
BenchmarkJSONMetadata_UnmarshalJSON-4   	 1000000	      1773 ns/op	     440 B/op	      12 allocs/op
```

Author: boekkooi-fresh

Gopkg.lock

@@ -1,6 +1,10 @@
 package metadata
 
-import "encoding/json"
+import (
+	"encoding/json"
+
+	"github.com/mailru/easyjson/jlexer"
+)
 
 // Metadata is an immutable map[string]interface{} implementation
 type Metadata interface {
@@ -117,12 +121,34 @@ func (j JSONMetadata) MarshalJSON() ([]byte, error) {
 
 // UnmarshalJSON unmarshal the json into Metdadata
 func (j *JSONMetadata) UnmarshalJSON(data []byte) error {
-	var valueMap map[string]interface{}
-	err := json.Unmarshal(data, &valueMap)
-	if err != nil {
-		return err
+	r := jlexer.Lexer{Data: data}
+	j.UnmarshalEasyJSON(&r)
+	return r.Error()
+}
+
+// UnmarshalEasyJSON supports easyjson.Unmarshaler interface
+func (j *JSONMetadata) UnmarshalEasyJSON(in *jlexer.Lexer) {
+	metadata := New()
+
+	isTopLevel := in.IsStart()
+	if in.IsNull() {
+		if isTopLevel {
+			in.Consumed()
+		}
+		in.Skip()
+		return
+	}
+	in.Delim('{')
+	for !in.IsDelim('}') {
+		key := in.UnsafeString()
+		in.WantColon()
+		metadata = WithValue(metadata, key, in.Interface())
+		in.WantComma()
+	}
+	in.Delim('}')
+	if isTopLevel {
+		in.Consumed()
 	}
 
-	j.Metadata = FromMap(valueMap)
-	return nil
+	j.Metadata = metadata
 }

metadata/metadata.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/hellofresh/goengine/metadata"
+	"github.com/mailru/easyjson"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -252,3 +253,16 @@ func BenchmarkJSONMetadata_UnmarshalJSON(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkJSONMetadata_UnmarshalEasyJSON(b *testing.B) {
+	payload := []byte(`{"_aggregate_id": "b9ebca7a-c1eb-40dd-94a4-fac7c5e84fb5", "_aggregate_type": "bank_account", "_aggregate_version": 1}`)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		var m metadata.JSONMetadata
+		err := easyjson.Unmarshal(payload, &m)
+		if err != nil {
+			b.Fail()
+		}
+	}
+}

metadata/metadata_test.go

@@ -2,14 +2,14 @@ package sql
 
 import (
 	"database/sql"
-	"encoding/json"
 	"fmt"
 	"time"
 
 	"github.com/hellofresh/goengine"
 	"github.com/hellofresh/goengine/aggregate"
 	driverSQL "github.com/hellofresh/goengine/driver/sql"
 	"github.com/hellofresh/goengine/metadata"
+	"github.com/mailru/easyjson"
 )
 
 // Ensure that AggregateChangedFactory satisfies the MessageFactory interface
@@ -79,7 +79,7 @@ func (a *aggregateChangedEventStream) Message() (goengine.Message, int64, error)
 	}
 
 	metadataWrapper := metadata.JSONMetadata{Metadata: metadata.New()}
-	if err := json.Unmarshal(jsonMetadata, &metadataWrapper); err != nil {
+	if err := easyjson.Unmarshal(jsonMetadata, &metadataWrapper); err != nil {
 		return nil, 0, err
 	}
 	meta := metadataWrapper.Metadata

strategy/json/sql/message_factory_aggregate.go

@@ -164,7 +164,7 @@ func TestAggregateChangedFactory_CreateFromRows(t *testing.T) {
 
 					return mockRows, mocks.NewMessagePayloadFactory(ctrl)
 				},
-				"unexpected end of JSON input",
+				"parse error: expected { near offset 1 of ''",
 			},
 			{
 				"bad payload",