Add ability to support strict JSON unmarshal for limits

This is the default behaviour for YAML now.
```
	in := `{"unknown_fields": 100}`
	l := cortex.Limits{}

	fmt.Println(yaml.UnmarshalStrict([]byte(in), &l))

	// yaml: unmarshal errors:
	// line 1: field unknown_fields not found in type validation.plain
```

This PR adds same behaviour if unmarshalled from JSON input as well.

Signed-off-by: Kaviraj <[email protected]>

Author: kavirajk

pkg/util/validation/limits.go

@@ -1,6 +1,7 @@
 package validation
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"flag"
@@ -229,7 +230,10 @@ func (l *Limits) UnmarshalJSON(data []byte) error {
 	}
 
 	type plain Limits
-	return json.Unmarshal(data, (*plain)(l))
+	dec := json.NewDecoder(bytes.NewReader(data))
+	dec.DisallowUnknownFields()
+
+	return dec.Decode((*plain)(l))
 }
 
 func (l *Limits) copyNotificationIntegrationLimits(defaults NotificationRateLimitMap) {

pkg/util/validation/limits_test.go

@@ -3,6 +3,7 @@ package validation
 import (
 	"encoding/json"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -165,6 +166,15 @@ func TestLimitsLoadingFromJson(t *testing.T) {
 
 	assert.Equal(t, 0.5, l.IngestionRate, "from json")
 	assert.Equal(t, 100, l.MaxLabelNameLength, "from defaults")
+
+	// Unmarshal should fail if input contains unknown struct fields and
+	// the decoder flag `json.Decoder.DisallowUnknownFields()` is set
+	inp = `{"unknown_fields": 100}`
+	l = Limits{}
+	dec := json.NewDecoder(strings.NewReader(inp))
+	dec.DisallowUnknownFields()
+	err = dec.Decode(&l)
+	assert.Error(t, err)
 }
 
 func TestLimitsTagsYamlMatchJson(t *testing.T) {