metadata: Define MD type & impl Format method

Define the type to store a metadata unit for the error and implement
its Format method with its test.

metadata.go

@@ -0,0 +1,19 @@
+package errors
+
+import "fmt"
+
+// MD is a key/value pair to add metadata to an error.
+type MD struct {
+	K string
+	V interface{}
+}
+
+// Format satisfies the fmt.Formatter interface.
+// It only prints when 'v' verb is used.
+func (md MD) Format(state fmt.State, verb rune) {
+	if verb != 'v' {
+		return
+	}
+
+	_, _ = fmt.Fprintf(state, "{%q: %+v}", md.K, md.V)
+}

metadata_test.go

@@ -0,0 +1,59 @@
+package errors
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMD_Format(t *testing.T) {
+	var tcases = []struct {
+		desc   string
+		format string
+		md     MD
+		expout string
+	}{
+		{
+			desc:   "'v' verb",
+			format: "%v",
+			md:     MD{K: "a-key", V: "a-value"},
+			expout: "{\"a-key\": a-value}",
+		},
+		{
+			desc:   "'v' verb when value is a struct",
+			format: "%v",
+			md:     MD{K: "a-key", V: struct{ Name string }{Name: "Ivan"}},
+			expout: "{\"a-key\": {Name:Ivan}}",
+		},
+		{
+			desc:   "'v' verb with '+' flag",
+			format: "%+v",
+			md:     MD{K: "a key", V: 10.5},
+			expout: "{\"a key\": 10.5}",
+		},
+		{
+			desc: "any other verb",
+			format: func() string {
+				var verbs = [...]string{
+					"t", "b", "c", "d", "o", "q", "x", "X", "U", "e", "E", "f", "F", "g", "G", "q", "p", "s",
+				}
+
+				return fmt.Sprintf("%%%s", verbs[rand.Intn(len(verbs))])
+			}(),
+			md:     MD{K: "some-key", V: "some-value"},
+			expout: "",
+		},
+	}
+
+	for i := range tcases {
+		var tc = tcases[i]
+		t.Run(tc.desc, func(t *testing.T) {
+			t.Parallel()
+
+			var out = fmt.Sprintf(tc.format, tc.md)
+			assert.Equal(t, tc.expout, out)
+		})
+	}
+}