Remove optional part from zero_threshold (open-telemetry#453)

opentelemetry/proto/metrics/v1/metrics.proto

@@ -599,10 +599,10 @@ message ExponentialHistogramDataPoint {
   // ZeroThreshold may be optionally set to convey the width of the zero
   // region. Where the zero region is defined as the closed interval
   // [-ZeroThreshold, ZeroThreshold].
-  // When ZeroThreshold is unset or 0, zero count bucket stores
-  // values that cannot be expressed using the standard exponential formula as
-  // well as values that have been rounded to zero.
-  optional double zero_threshold = 14;
+  // When ZeroThreshold is 0, zero count bucket stores values that cannot be
+  // expressed using the standard exponential formula as well as values that
+  // have been rounded to zero.
+  double zero_threshold = 14;
 }
 
 // SummaryDataPoint is a single data point in a timeseries that describes the