Revert "promql: Remove unused Point.MarshalJSON method (#9860)" (#9864)

This reverts commit 427425cc9f.

Signed-off-by: Ganesh Vernekar <ganeshvern@gmail.com>

promql/value.go

@@ -77,9 +77,6 @@ func (s Series) String() string {
 }
 
 // Point represents a single data point for a given timestamp.
-//
-// Note that Point's JSON marshaling is done in an optimized fashion in
-// web/api/v1/api.go. Therefore, no MarshalJSON method is provided here.
 type Point struct {
 	T int64
 	V float64
@@ -90,6 +87,12 @@ func (p Point) String() string {
 	return fmt.Sprintf("%v @[%v]", v, p.T)
 }
 
+// MarshalJSON implements json.Marshaler.
+func (p Point) MarshalJSON() ([]byte, error) {
+	v := strconv.FormatFloat(p.V, 'f', -1, 64)
+	return json.Marshal([...]interface{}{float64(p.T) / 1000, v})
+}
+
 // Sample is a single sample belonging to a metric.
 type Sample struct {
 	Point