fix: Don’t consider log events as root spans (#1208)

## Which problem is this PR solving?

Log events should never be considered root spans even though they do not
contain a `trace.parent_id`. A log event that is part of a trace would
mark the trace complete as if it's considered a root span.

This change updates the check for whether a span is considered a root
span to also check for the meta field `meta.signal_type`. If the field
exists and is set to "log", we return false.

## Short description of the changes
- Update `InMemoryCollector.isRootSpan` to check if `meta.signal_type`
and has a value of "log"
- Add unit tests for InMemoryCollector to verify expected behaviour for
isRootSpan

collect/collect.go

@@ -575,6 +575,11 @@ func mergeTraceAndSpanSampleRates(sp *types.Span, traceSampleRate uint, dryRunMo
 }
 
 func (i *InMemCollector) isRootSpan(sp *types.Span) bool {
+	// log event should never be considered a root span, check for that first
+	if signalType, _ := sp.Data["meta.signal_type"]; signalType == "log" {
+		return false
+	}
+	// check if the event has a parent id using the configured parent id field names
 	for _, parentIdFieldName := range i.Config.GetParentIdFieldNames() {
 		parentId := sp.Data[parentIdFieldName]
 		if _, ok := parentId.(string); ok {

collect/collect_test.go

@@ -1364,3 +1364,66 @@ func TestSpanWithRuleReasons(t *testing.T) {
 	}
 	transmission.Mux.RUnlock()
 }
+
+func TestIsRootSpan(t *testing.T) {
+	tesCases := []struct {
+		name     string
+		span     *types.Span
+		expected bool
+	}{
+		{
+			name: "root span - no parent id",
+			span: &types.Span{
+				Event: types.Event{
+					Data: map[string]interface{}{},
+				},
+			},
+			expected: true,
+		},
+		{
+			name: "non-root span - empty parent id",
+			span: &types.Span{
+				Event: types.Event{
+					Data: map[string]interface{}{
+						"trace.parent_id": "",
+					},
+				},
+			},
+			expected: false,
+		},
+		{
+			name: "non-root span - parent id",
+			span: &types.Span{
+				Event: types.Event{
+					Data: map[string]interface{}{
+						"trace.parent_id": "some-id",
+					},
+				},
+			},
+			expected: false,
+		},
+		{
+			name: "non-root span - no parent id but has signal_type of log",
+			span: &types.Span{
+				Event: types.Event{
+					Data: map[string]interface{}{
+						"meta.signal_type": "log",
+					},
+				},
+			},
+			expected: false,
+		},
+	}
+
+	collector := &InMemCollector{
+		Config: &config.MockConfig{
+			ParentIdFieldNames: []string{"trace.parent_id", "parentId"},
+		},
+	}
+
+	for _, tc := range tesCases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.expected, collector.isRootSpan(tc.span))
+		})
+	}
+}