Add field of present flag to proto build (#366)

* Add field presence flag to proto build.

* Fix #314 - Cannot distinguish between sum being 0 and undefined.

* remove gogo from Makefile

Co-authored-by: Josh Suereth <joshuasuereth@google.com>

Makefile

@@ -19,7 +19,6 @@ gen-all: gen-cpp gen-csharp gen-go gen-java gen-kotlin gen-objc gen-openapi gen-
 
 OTEL_DOCKER_PROTOBUF ?= otel/build-protobuf:0.9.0
 PROTOC := docker run --rm -u ${shell id -u} -v${PWD}:${PWD} -w${PWD} ${OTEL_DOCKER_PROTOBUF} --proto_path=${PWD}
-PROTO_INCLUDES := -I/usr/include/github.com/gogo/protobuf
 
 PROTO_GEN_CPP_DIR ?= $(GENDIR)/cpp
 PROTO_GEN_CSHARP_DIR ?= $(GENDIR)/csharp
@@ -63,7 +62,7 @@ gen-csharp:
 gen-go:
 	rm -rf ./$(PROTO_GEN_GO_DIR)
 	mkdir -p ./$(PROTO_GEN_GO_DIR)
-	$(foreach file,$(PROTO_FILES),$(call exec-command,$(PROTOC) $(PROTO_INCLUDES) --gogo_out=plugins=grpc:./$(PROTO_GEN_GO_DIR) $(file)))
+	$(foreach file,$(PROTO_FILES),$(call exec-command,$(PROTOC) --go_out=plugins=grpc:./$(PROTO_GEN_GO_DIR) $(file)))
 	$(PROTOC) --grpc-gateway_out=logtostderr=true,grpc_api_configuration=opentelemetry/proto/collector/trace/v1/trace_service_http.yaml:./$(PROTO_GEN_GO_DIR) opentelemetry/proto/collector/trace/v1/trace_service.proto
 	$(PROTOC) --grpc-gateway_out=logtostderr=true,grpc_api_configuration=opentelemetry/proto/collector/metrics/v1/metrics_service_http.yaml:./$(PROTO_GEN_GO_DIR) opentelemetry/proto/collector/metrics/v1/metrics_service.proto
 	$(PROTOC) --grpc-gateway_out=logtostderr=true,grpc_api_configuration=opentelemetry/proto/collector/logs/v1/logs_service_http.yaml:./$(PROTO_GEN_GO_DIR) opentelemetry/proto/collector/logs/v1/logs_service.proto

opentelemetry/proto/metrics/v1/metrics.proto

@@ -459,7 +459,7 @@ message HistogramDataPoint {
   // Negative events *can* be recorded, but sum should not be filled out when
   // doing so.  This is specifically to enforce compatibility w/ OpenMetrics,
   // see: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#histogram
-  double sum = 5;
+  optional double sum = 5;
 
   // bucket_counts is an optional field contains the count values of histogram
   // for each bucket.