[key] add Stringer and Infer methods (#662)

* add Stringer and Infer methods

* fix nit

* Update api/key/key.go

Co-Authored-By: Tyler Yahn <MrAlias@users.noreply.github.com>

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

api/key/key.go

@@ -15,6 +15,8 @@
 package key
 
 import (
+	"fmt"
+
 	"go.opentelemetry.io/otel/api/core"
 )
 
@@ -71,6 +73,12 @@ func String(k, v string) core.KeyValue {
 	return New(k).String(v)
 }
 
+// Stringer creates a new key-value pair with a passed name and a string
+// value generated by the passed Stringer interface.
+func Stringer(k string, v fmt.Stringer) core.KeyValue {
+	return New(k).String(v.String())
+}
+
 // Int creates a new key-value pair instance with a passed name and
 // either an int32 or an int64 value, depending on whether the int
 // type is 32 or 64 bits wide.
@@ -84,3 +92,34 @@ func Int(k string, v int) core.KeyValue {
 func Uint(k string, v uint) core.KeyValue {
 	return New(k).Uint(v)
 }
+
+// Infer creates a new key-value pair instance with a passed name and
+// automatic type inference. This is slower, and not type-safe.
+func Infer(k string, value interface{}) core.KeyValue {
+	switch v := value.(type) {
+	case bool:
+		return Bool(k, v)
+	case int:
+		return Int(k, v)
+	case uint:
+		return Uint(k, v)
+	case int32:
+		return Int32(k, v)
+	case int64:
+		return Int64(k, v)
+	case uint32:
+		return Uint32(k, v)
+	case uint64:
+		return Uint64(k, v)
+	case float32:
+		return Float32(k, v)
+	case float64:
+		return Float64(k, v)
+	case string:
+		return String(k, v)
+	case fmt.Stringer:
+		return Stringer(k, v)
+	default:
+		return String(k, fmt.Sprint(v))
+	}
+}

api/key/key_test.go

@@ -15,6 +15,7 @@
 package key_test
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -119,3 +120,85 @@ func TestKeyValueConstructors(t *testing.T) {
 		})
 	}
 }
+
+func TestInfer(t *testing.T) {
+	builder := &strings.Builder{}
+	builder.WriteString("foo")
+	for _, testcase := range []struct {
+		key       string
+		value     interface{}
+		wantType  core.ValueType
+		wantValue interface{}
+	}{
+		{
+			key:       "bool type inferred",
+			value:     true,
+			wantType:  core.BOOL,
+			wantValue: true,
+		},
+		{
+			key:       "int64 type inferred",
+			value:     int64(42),
+			wantType:  core.INT64,
+			wantValue: int64(42),
+		},
+		{
+			key:       "uint64 type inferred",
+			value:     uint64(42),
+			wantType:  core.UINT64,
+			wantValue: uint64(42),
+		},
+		{
+			key:       "float64 type inferred",
+			value:     float64(42.1),
+			wantType:  core.FLOAT64,
+			wantValue: 42.1,
+		},
+		{
+			key:       "int32 type inferred",
+			value:     int32(42),
+			wantType:  core.INT32,
+			wantValue: int32(42),
+		},
+		{
+			key:       "uint32 type inferred",
+			value:     uint32(42),
+			wantType:  core.UINT32,
+			wantValue: uint32(42),
+		},
+		{
+			key:       "float32 type inferred",
+			value:     float32(42.1),
+			wantType:  core.FLOAT32,
+			wantValue: float32(42.1),
+		},
+		{
+			key:       "string type inferred",
+			value:     "foo",
+			wantType:  core.STRING,
+			wantValue: "foo",
+		},
+		{
+			key:       "stringer type inferred",
+			value:     builder,
+			wantType:  core.STRING,
+			wantValue: "foo",
+		},
+		{
+			key:       "unknown value serialized as %v",
+			value:     nil,
+			wantType:  core.STRING,
+			wantValue: "<nil>",
+		},
+	} {
+		t.Logf("Running test case %s", testcase.key)
+		kv := key.Infer(testcase.key, testcase.value)
+		if kv.Value.Type() != testcase.wantType {
+			t.Errorf("wrong value type, got %#v, expected %#v", kv.Value.Type(), testcase.wantType)
+		}
+		got := kv.Value.AsInterface()
+		if diff := cmp.Diff(testcase.wantValue, got); diff != "" {
+			t.Errorf("+got, -want: %s", diff)
+		}
+	}
+}