jwt: benchmark Parse (#2958)

```
$ benchstat <(go test ./jwt -run=NONE -bench=BenchmarkParse -count=10)
goos: linux
goarch: amd64
pkg: github.com/zalando/skipper/jwt
cpu: Intel(R) Core(TM) i5-8350U CPU @ 1.70GHz
        │ /dev/fd/63  │
        │   sec/op    │
Parse-8   13.53µ ± 8%

        │  /dev/fd/63  │
        │     B/op     │
Parse-8   1.656Ki ± 0%

        │ /dev/fd/63 │
        │ allocs/op  │
Parse-8   40.00 ± 0%
```

Signed-off-by: Alexander Yastrebov <[email protected]>

Author: AlexanderYastrebov

jwt/token_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -57,10 +58,38 @@ func TestParse(t *testing.T) {
 	}
 }
 
-func marshalBase64JSON(t *testing.T, v interface{}) string {
+func marshalBase64JSON(tb testing.TB, v interface{}) string {
 	d, err := json.Marshal(v)
 	if err != nil {
-		t.Fatalf("failed to marshal json: %v, %v", v, err)
+		tb.Fatalf("failed to marshal json: %v, %v", v, err)
 	}
 	return base64.RawURLEncoding.EncodeToString(d)
 }
+
+var parseSink *Token
+
+func BenchmarkParse(b *testing.B) {
+	claims := map[string]interface{}{
+		"azp":                    strings.Repeat("z", 100),
+		"exp":                    1234567890,
+		"aaaaaaaaaaaaaaaaaaaaaa": strings.Repeat("a", 40),
+		"bbbbbbbbbbbbbbbbbbbbbb": strings.Repeat("b", 40),
+		"cccccccccccccccccccccc": strings.Repeat("c", 40),
+		"iat":                    1234567890,
+		"iss":                    "https://skipper.identity.example.org",
+		"sub":                    "foo_bar-baz-qux",
+	}
+
+	value := strings.Repeat("x", 64) + "." + marshalBase64JSON(b, claims) + "." + strings.Repeat("x", 128)
+
+	_, err := Parse(value)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		parseSink, _ = Parse(value)
+	}
+}