[OTLP] Fix WritePackedLength

Fix `fieldNumber` not being used.

Author: martincostello

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Serializer/ProtobufOtlpMetricSerializer.cs

@@ -573,7 +573,7 @@ static int WritePackedLength(byte[] buffer, int writePosition, int length, int f
                 buffer,
                 writePosition,
                 length * 8,
-                ProtobufOtlpMetricFieldNumberConstants.HistogramDataPoint_Bucket_Counts,
+                fieldNumber,
                 ProtobufWireType.LEN);
         }
     }