Remove tag clean up

Removing the _dd.span_events.has_exception tag requires to loop through
the whole chunk which is not ideal. Also:
- tag is hidden (although visible in devtools)
- indication of why the chunk was sampled if there is no error span
- error sampler can be disabled so no clean up
And while it is there, this tag could also be used at processing to
detect exception span events without even unmarshalling.

pkg/trace/agent/agent.go

@@ -641,7 +641,6 @@ func (a *Agent) runSamplers(now time.Time, ts *info.TagStats, pt traceutil.Proce
 			return true, true
 		}
 		if traceContainsError(pt.TraceChunk.Spans) {
-			cleanUpExceptionSpanEventTag(pt.TraceChunk.Spans)
 			return a.ErrorsSampler.Sample(now, pt.TraceChunk.Spans, pt.Root, pt.TracerEnv), true
 		}
 		return false, true
@@ -679,7 +678,6 @@ func (a *Agent) runSamplers(now time.Time, ts *info.TagStats, pt traceutil.Proce
 	}
 
 	if traceContainsError(pt.TraceChunk.Spans) {
-		cleanUpExceptionSpanEventTag(pt.TraceChunk.Spans)
 		return a.ErrorsSampler.Sample(now, pt.TraceChunk.Spans, pt.Root, pt.TracerEnv), true
 	}
 
@@ -706,12 +704,6 @@ func spanContainsExceptionSpanEvent(span *pb.Span) bool {
 	return false
 }
 
-func cleanUpExceptionSpanEventTag(trace pb.Trace) {
-	for _, span := range trace {
-		delete(span.Meta, "_dd.span_events.has_exception")
-	}
-}
-
 func filteredByTags(root *pb.Span, require, reject []*config.Tag, requireRegex, rejectRegex []*config.TagRegex) bool {
 	for _, tag := range reject {
 		if v, ok := root.Meta[tag.K]; ok && (tag.V == "" || v == tag.V) {

pkg/trace/agent/agent_test.go

@@ -1353,16 +1353,11 @@ func TestSampleTrace(t *testing.T) {
 			conf:              cfg,
 		}
 		t.Run(name, func(t *testing.T) {
-			_, hasExceptionSpanEvent := tt.trace.TraceChunk.Spans[0].Meta["_dd.span_events.has_exception"]
 			keep, _ := a.traceSampling(now, info.NewReceiverStats().GetTagStats(info.Tags{}), &tt.trace)
 			assert.Equal(t, tt.keep, keep)
 			assert.Equal(t, !tt.keep, tt.trace.TraceChunk.DroppedTrace)
 			cfg.Features["error_rare_sample_tracer_drop"] = struct{}{}
 			defer delete(cfg.Features, "error_rare_sample_tracer_drop")
-			if hasExceptionSpanEvent {
-				// element was cleaned up during sampling
-				tt.trace.TraceChunk.Spans[0].Meta["_dd.span_events.has_exception"] = "true"
-			}
 			keep, _ = a.traceSampling(now, info.NewReceiverStats().GetTagStats(info.Tags{}), &tt.trace)
 			assert.Equal(t, tt.keepWithFeature, keep)
 			assert.Equal(t, !tt.keepWithFeature, tt.trace.TraceChunk.DroppedTrace)