supports marshaling values as json (#948)

CHANGELOG.md

@@ -12,7 +12,11 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - Github action to generate protobuf Go bindings locally in `internal/opentelemetry-proto-gen`. (#938)
 - OTLP .proto files from `open-telemetry/opentelemetry-proto` imported as a git submodule under `internal/opentelemetry-proto`.
-   References to `github.com/open-telemetry/opentelemetry-proto` changed to `go.opentelemetry.io/otel/internal/opentelemetry-proto-gen`. (#948)
+   References to `github.com/open-telemetry/opentelemetry-proto` changed to `go.opentelemetry.io/otel/internal/opentelemetry-proto-gen`. (#942)
+
+### Changed
+
+- Non-nil value structs for key-value pairs will be marshalled using JSON rather than Sprintf. (#948)
 
 ### Removed
 

api/kv/kv.go

@@ -15,6 +15,7 @@
 package kv
 
 import (
+	"encoding/json"
 	"fmt"
 	"reflect"
 
@@ -138,5 +139,8 @@ func Infer(k string, value interface{}) KeyValue {
 	case reflect.String:
 		return String(k, rv.String())
 	}
+	if b, err := json.Marshal(value); value != nil && err == nil {
+		return String(k, string(b))
+	}
 	return String(k, fmt.Sprint(value))
 }

api/kv/kv_test.go

@@ -124,6 +124,17 @@ func TestKeyValueConstructors(t *testing.T) {
 func TestInfer(t *testing.T) {
 	builder := &strings.Builder{}
 	builder.WriteString("foo")
+	jsonifyStruct := struct {
+		Public    string
+		private   string
+		Tagged    string `json:"tagName"`
+		Empty     string
+		OmitEmpty string `json:",omitempty"`
+		Omit      string `json:"-"`
+	}{"foo", "bar", "baz", "", "", "omitted"}
+	invalidStruct := struct {
+		N complex64
+	}{complex(0, 0)}
 	for _, testcase := range []struct {
 		key       string
 		value     interface{}
@@ -190,6 +201,18 @@ func TestInfer(t *testing.T) {
 			wantType:  value.STRING,
 			wantValue: "<nil>",
 		},
+		{
+			key:       "JSON struct serialized correctly",
+			value:     &jsonifyStruct,
+			wantType:  value.STRING,
+			wantValue: `{"Public":"foo","tagName":"baz","Empty":""}`,
+		},
+		{
+			key:       "Invalid JSON struct falls back to string",
+			value:     &invalidStruct,
+			wantType:  value.STRING,
+			wantValue: "&{(0+0i)}",
+		},
 	} {
 		t.Logf("Running test case %s", testcase.key)
 		keyValue := kv.Infer(testcase.key, testcase.value)