json marshal for Extrema type, makefile typo fix

Makefile

@@ -192,7 +192,7 @@ test-coverage: | $(GOCOVMERGE)
 	done; \
 	$(GOCOVMERGE) $$(find . -name coverage.out) > coverage.txt
 
-# Adding a directory will include all benchmarks in that direcotry if a filter is not specified.
+# Adding a directory will include all benchmarks in that directory if a filter is not specified.
 BENCHMARK_TARGETS := sdk/trace
 .PHONY: benchmark
 benchmark: $(BENCHMARK_TARGETS:%=benchmark/%)

sdk/metric/metricdata/data.go

@@ -15,6 +15,8 @@
 package metricdata // import "go.opentelemetry.io/otel/sdk/metric/metricdata"
 
 import (
+	"encoding/json"
+	"fmt"
 	"time"
 
 	"go.opentelemetry.io/otel/attribute"
@@ -211,6 +213,14 @@ type Extrema[N int64 | float64] struct {
 	valid bool
 }
 
+// MarshalJson converts the Extrema value to JSON number
+func (e *Extrema[N]) MarshalJSON() ([]byte, error) {
+	if !e.valid {
+		return []byte("0"), nil
+	}
+	return []byte(json.Number(fmt.Sprint(e.value))), nil
+}
+
 // NewExtrema returns an Extrema set to v.
 func NewExtrema[N int64 | float64](v N) Extrema[N] {
 	return Extrema[N]{value: v, valid: true}

sdk/metric/metricdata/metricdatatest/assertion_test.go

@@ -15,6 +15,7 @@
 package metricdatatest // import "go.opentelemetry.io/otel/sdk/metric/metricdata/metricdatatest"
 
 import (
+	"encoding/json"
 	"testing"
 	"time"
 
@@ -164,6 +165,8 @@ var (
 	minFloat64C              = metricdata.NewExtrema(-1.)
 	minInt64C                = metricdata.NewExtrema[int64](-1)
 
+	minFloat64D = metricdata.NewExtrema(-9.999999)
+
 	histogramDataPointInt64A = metricdata.HistogramDataPoint[int64]{
 		Attributes:   attrA,
 		StartTime:    startA,
@@ -1010,3 +1013,22 @@ func TestAssertAttributesFail(t *testing.T) {
 	}
 	assert.False(t, AssertHasAttributes(fakeT, sum, attribute.Bool("A", true)))
 }
+
+func AssertMarshal[N int64 | float64](t *testing.T, expected string, i *metricdata.Extrema[N]) {
+	t.Helper()
+
+	b, err := json.Marshal(i)
+	assert.NoError(t, err)
+	assert.Equal(t, expected, string(b))
+}
+
+func TestAssertMarshal(t *testing.T) {
+	AssertMarshal(t, "-1", &minFloat64A)
+	AssertMarshal(t, "3", &minFloat64B)
+	AssertMarshal(t, "-9.999999", &minFloat64D)
+	AssertMarshal(t, "99", &maxFloat64B)
+
+	AssertMarshal(t, "-1", &minInt64A)
+	AssertMarshal(t, "3", &minInt64B)
+	AssertMarshal(t, "99", &maxInt64B)
+}