enhance: capture timestamp earlier for more accurate span timing

Capture the timestamp as close as posssible to the moment a span is
created. This improves the accuracy of both the start time and duration
measurements.

Author: dmehala

src/datadog/span.cpp

@@ -28,6 +28,7 @@ Span::Span(SpanData* data, const std::shared_ptr<TraceSegment>& trace_segment,
 }
 
 Span::~Span() {
+  const auto now = clock_();
   if (!trace_segment_) {
     // We were moved from.
     return;
@@ -36,16 +37,17 @@ Span::~Span() {
   if (end_time_) {
     data_->duration = *end_time_ - data_->start.tick;
   } else {
-    const auto now = clock_();
     data_->duration = now - data_->start;
   }
 
   trace_segment_->span_finished();
 }
 
 Span Span::create_child(const SpanConfig& config) const {
+  auto now = clock_();
   auto span_data = std::make_unique<SpanData>();
-  span_data->apply_config(trace_segment_->defaults(), config, clock_);
+  span_data->start = now;
+  span_data->apply_config(trace_segment_->defaults(), config);
   span_data->trace_id = data_->trace_id;
   span_data->parent_id = data_->span_id;
   span_data->span_id = generate_span_id_();

src/datadog/span_data.cpp

@@ -36,7 +36,11 @@ Optional<StringView> SpanData::version() const {
 }
 
 void SpanData::apply_config(const SpanDefaults& defaults,
-                            const SpanConfig& config, const Clock& clock) {
+                            const SpanConfig& config) {
+  if (config.start) {
+    start = *config.start;
+  }
+
   std::string version;
   if (config.service) {
     service = *config.service;
@@ -66,11 +70,6 @@ void SpanData::apply_config(const SpanDefaults& defaults,
 
   resource = config.resource.value_or(name);
   service_type = config.service_type.value_or(defaults.service_type);
-  if (config.start) {
-    start = *config.start;
-  } else {
-    start = clock();
-  }
 }
 
 Expected<void> msgpack_encode(std::string& destination, const SpanData& span) {

src/datadog/span_data.h

@@ -39,10 +39,8 @@ struct SpanData {
 
   // Modify the properties of this object to honor the specified `config` and
   // `defaults`.  The properties of `config`, if set, override the properties of
-  // `defaults`. Use the specified `clock` to provide a start none of none is
-  // specified in `config`.
-  void apply_config(const SpanDefaults& defaults, const SpanConfig& config,
-                    const Clock& clock);
+  // `defaults`.
+  void apply_config(const SpanDefaults& defaults, const SpanConfig& config);
 };
 
 // Append to the specified `destination` the MessagePack representation of the

src/datadog/tracer.cpp

@@ -168,9 +168,11 @@ void Tracer::store_config() {
 Span Tracer::create_span() { return create_span(SpanConfig{}); }
 
 Span Tracer::create_span(const SpanConfig& config) {
+  auto now = clock_();
   auto defaults = config_manager_->span_defaults();
   auto span_data = std::make_unique<SpanData>();
-  span_data->apply_config(*defaults, config, clock_);
+  span_data->start = now;
+  span_data->apply_config(*defaults, config);
   span_data->trace_id = generator_->trace_id(span_data->start);
   span_data->span_id = span_data->trace_id.low;
   span_data->parent_id = 0;
@@ -203,6 +205,7 @@ Expected<Span> Tracer::extract_span(const DictReader& reader) {
 
 Expected<Span> Tracer::extract_span(const DictReader& reader,
                                     const SpanConfig& config) {
+  auto now = clock_();
   assert(!extraction_styles_.empty());
 
   AuditedReader audited_reader{reader};
@@ -335,7 +338,8 @@ Expected<Span> Tracer::extract_span(const DictReader& reader,
 
   // We're done extracting fields.  Now create the span.
   // This is similar to what we do in `create_span`.
-  span_data->apply_config(*config_manager_->span_defaults(), config, clock_);
+  span_data->start = now;
+  span_data->apply_config(*config_manager_->span_defaults(), config);
   span_data->span_id = generator_->span_id();
   span_data->trace_id = *merged_context.trace_id;
   span_data->parent_id = *merged_context.parent_id;