commenting it up trying to figure out how to move sampling around

lib/datadog/tracing/distributed/propagation.rb

@@ -62,6 +62,12 @@ def inject!(digest, data)
           end
 
           result = false
+          # Probably sample here
+          # Do we need to update tags on the parent span here first?
+          # That should probably be done in the tracer, not here.
+          # Can we sample off of digest or do we need to grab the parent span?
+          # Looking at how we do span sampling would be helpful to answer some of this.
+          components.sampler.sample!(digest)
 
           # Inject all configured propagation styles
           @propagation_style_inject.each do |propagator|

lib/datadog/tracing/tracer.rb

@@ -44,7 +44,8 @@ class Tracer
       # @param default_service [String] A fallback value for {Datadog::Tracing::Span#service}, as spans without
       #                        service are rejected
       # @param enabled [Boolean] set if the tracer submits or not spans to the local agent
-      # @param sampler [Datadog::Tracing::Sampler] a tracer sampler, responsible for filtering out spans when needed
+      # @param
+      sampler [Datadog::Tracing::Sampler] a tracer sampler, responsible for filtering out spans when needed
       # @param tags [Hash] default tags added to all spans
       # @param writer [Datadog::Tracing::Writer] consumes traces returned by the provided +trace_flush+
       def initialize(
@@ -317,7 +318,7 @@ def build_trace(digest = nil)
         # Resolve hostname if configured
         hostname = Core::Environment::Socket.hostname if Datadog.configuration.tracing.report_hostname
         hostname = hostname && !hostname.empty? ? hostname : nil
-
+        # So we can build traces out of digests
         if digest
           TraceOperation.new(
             hostname: hostname,
@@ -342,15 +343,23 @@ def build_trace(digest = nil)
       end
 
       def bind_trace_events!(trace_op)
+        # need to find out what trace_op.send does
+        # ok trace_op is just a trace and we run bind_trace_events right after
+        # build_trace
+        # and what events is.
+        # events is probably just all the spans
         events = trace_op.send(:events)
-
+        # so we loop through all the spans and trace sample them if they're the root span
+        # and span sample all of them as well.
         events.span_before_start.subscribe do |event_span_op, event_trace_op|
           event_trace_op.service ||= @default_service
           event_span_op.service ||= @default_service
+          # here's where we sample for the trace rn
           sample_trace(event_trace_op) if event_span_op && event_span_op.parent_id == 0
         end
 
         events.span_finished.subscribe do |event_span, event_trace_op|
+          # then span sampling is done here
           sample_span(event_trace_op, event_span)
           flush_trace(event_trace_op)
         end