change comment that allows timestamps to be optional

Signed-off-by: Callum Styan <callumstyan@gmail.com>

prompb/io/prometheus/write/v2/types.proto

@@ -107,15 +107,10 @@ message Exemplar {
   // value represents an exact example value. This can be useful when the exemplar
   // is attached to a histogram, which only gives an estimated value through buckets.
   double value = 2;
-  // timestamp represents an optional timestamp of the sample in ms.
+  // timestamp represents the timestamp of the exemplar in ms.
   //
   // For Go, see github.com/prometheus/prometheus/model/timestamp/timestamp.go
   // for conversion from/to time.Time to Prometheus timestamp.
-  //
-  // Note that the "optional" keyword is omitted due to
-  // https://cloud.google.com/apis/design/design_patterns.md#optional_primitive_fields
-  // Zero value means value not set. If you need to use exactly zero value for
-  // the timestamp, use 1 millisecond before or after.
   int64 timestamp = 3;
 }