Use json for writing floats as strings (open-telemetry#4934)

* use json for writing floats as strings

* avoid using reflection for floats

* extra unit test

* fix rebase

Co-authored-by: Bogdan Drutu <bogdandrutu@gmail.com>

CHANGELOG.md

@@ -22,6 +22,7 @@
 
 - Remove `Type` funcs in pdata (#4933)
 - Remove all deprecated funcs/structs from v0.46.0 (#4995)
+- AsString for pdata.AttributeValue now returns the JSON-encoded string of floats. (#4934)
 
 ### 🚩 Deprecations 🚩
 

model/internal/pdata/common.go

@@ -22,6 +22,7 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
+	"math"
 	"sort"
 	"strconv"
 
@@ -379,7 +380,7 @@ func (v Value) AsString() string {
 		return strconv.FormatBool(v.BoolVal())
 
 	case ValueTypeDouble:
-		return strconv.FormatFloat(v.DoubleVal(), 'f', -1, 64)
+		return float64AsString(v.DoubleVal())
 
 	case ValueTypeInt:
 		return strconv.FormatInt(v.IntVal(), 10)
@@ -400,6 +401,37 @@ func (v Value) AsString() string {
 	}
 }
 
+// See https://cs.opensource.google/go/go/+/refs/tags/go1.17.7:src/encoding/json/encode.go;l=585.
+// This allows us to avoid using reflection.
+func float64AsString(f float64) string {
+	if math.IsInf(f, 0) || math.IsNaN(f) {
+		return fmt.Sprintf("json: unsupported value: %s", strconv.FormatFloat(f, 'g', -1, int(64)))
+	}
+
+	// Convert as if by ES6 number to string conversion.
+	// This matches most other JSON generators.
+	// See golang.org/issue/6384 and golang.org/issue/14135.
+	// Like fmt %g, but the exponent cutoffs are different
+	// and exponents themselves are not padded to two digits.
+	scratch := [64]byte{}
+	b := scratch[:0]
+	abs := math.Abs(f)
+	fmt := byte('f')
+	if abs != 0 && (abs < 1e-6 || abs >= 1e21) {
+		fmt = 'e'
+	}
+	b = strconv.AppendFloat(b, f, fmt, -1, int(64))
+	if fmt == 'e' {
+		// clean up e-09 to e-9
+		n := len(b)
+		if n >= 4 && b[n-4] == 'e' && b[n-3] == '-' && b[n-2] == '0' {
+			b[n-2] = b[n-1]
+			b = b[:n-1]
+		}
+	}
+	return string(b)
+}
+
 func newAttributeKeyValueString(k string, v string) otlpcommon.KeyValue {
 	orig := otlpcommon.KeyValue{Key: k}
 	akv := Value{&orig.Value}

model/internal/pdata/common_test.go

@@ -17,6 +17,7 @@ package pdata
 import (
 	"encoding/base64"
 	"fmt"
+	"math"
 	"strconv"
 	"testing"
 
@@ -780,6 +781,15 @@ func BenchmarkAttributeValue_SetIntVal(b *testing.B) {
 	}
 }
 
+func BenchmarkAttributeValueFloat_AsString(b *testing.B) {
+	av := NewValueDouble(2359871345.583429543)
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		av.AsString()
+	}
+}
+
 func BenchmarkAttributeMap_Range(b *testing.B) {
 	const numElements = 20
 	rawOrig := make([]otlpcommon.KeyValue, numElements)
@@ -1023,6 +1033,16 @@ func TestAsString(t *testing.T) {
 			input:    NewValueDouble(1.61803399),
 			expected: "1.61803399",
 		},
+		{
+			name:     "small float64",
+			input:    NewValueDouble(.000000009),
+			expected: "9e-9",
+		},
+		{
+			name:     "bad float64",
+			input:    NewValueDouble(math.Inf(1)),
+			expected: "json: unsupported value: +Inf",
+		},
 		{
 			name:     "boolean",
 			input:    NewValueBool(true),