feat: Explain cid & field attributes for dagScanNode (#598)

## Relevant issue(s)
Resolves #597  

## Description
Adds two more attributes to be included in the explain graph for `dagScanNode`.

query/graphql/planner/dagscan.go

@@ -226,7 +226,23 @@ func (n *dagScanNode) Source() planNode { return n.headset }
 // Explain method returns a map containing all attributes of this node that
 // are to be explained, subscribes / opts-in this node to be an explainablePlanNode.
 func (n *dagScanNode) Explain() (map[string]interface{}, error) {
-	// explain the spans attribute.
+	explainerMap := map[string]interface{}{}
+
+	// Add the field attribute to the explaination if it exists.
+	if len(n.field) != 0 {
+		explainerMap["field"] = n.field
+	} else {
+		explainerMap["field"] = nil
+	}
+
+	// Add the cid attribute to the explaination if it exists.
+	if n.cid != nil && n.cid.Defined() {
+		explainerMap["cid"] = n.cid.Bytes()
+	} else {
+		explainerMap["cid"] = nil
+	}
+
+	// Build the explaination of the spans attribute.
 	spansExplainer := []map[string]interface{}{}
 	// Note: n.headset is `nil` for single commit selection query, so must check for it.
 	if n.headset != nil && n.headset.spans.HasValue {
@@ -240,10 +256,10 @@ func (n *dagScanNode) Explain() (map[string]interface{}, error) {
 			)
 		}
 	}
+	// Add the built spans attribute, if it was valid.
+	explainerMap[spansLabel] = spansExplainer
 
-	return map[string]interface{}{
-		spansLabel: spansExplainer,
-	}, nil
+	return explainerMap, nil
 }
 
 func (n *dagScanNode) Next() (bool, error) {

tests/integration/query/explain/dagscan_test.go

@@ -55,6 +55,8 @@ func TestExplainAllCommitsDagScan(t *testing.T) {
 							"filter": nil,
 							"commitSelectNode": dataMap{
 								"dagScanNode": dataMap{
+									"cid":   nil,
+									"field": "1",
 									"spans": []dataMap{
 										{
 											"start": "/bae-41598f0c-19bc-5da6-813b-e80f14a10df3/1",
@@ -112,6 +114,8 @@ func TestExplainAllCommitsDagScanWithoutField(t *testing.T) {
 							"filter": nil,
 							"commitSelectNode": dataMap{
 								"dagScanNode": dataMap{
+									"cid":   nil,
+									"field": "C",
 									"spans": []dataMap{
 										{
 											"start": "/bae-41598f0c-19bc-5da6-813b-e80f14a10df3/C",
@@ -208,6 +212,8 @@ func TestExplainLatestCommitsDagScan(t *testing.T) {
 							"filter": nil,
 							"commitSelectNode": dataMap{
 								"dagScanNode": dataMap{
+									"cid":   nil,
+									"field": "1",
 									"spans": []dataMap{
 										{
 											"start": "/bae-41598f0c-19bc-5da6-813b-e80f14a10df3/1",
@@ -266,6 +272,8 @@ func TestExplainLatestCommitsDagScanWithoutField(t *testing.T) {
 							"filter": nil,
 							"commitSelectNode": dataMap{
 								"dagScanNode": dataMap{
+									"cid":   nil,
+									"field": "C",
 									"spans": []dataMap{
 										{
 											"start": "/bae-41598f0c-19bc-5da6-813b-e80f14a10df3/C",
@@ -363,6 +371,45 @@ func TestExplainOneCommitDagScan(t *testing.T) {
 							"filter": nil,
 							"commitSelectNode": dataMap{
 								"dagScanNode": dataMap{
+									"cid": []uint8{
+										0x01,
+										0x70,
+										0x12,
+										0x20,
+										0x98,
+										0x96,
+										0xeb,
+										0xf9,
+										0xa1,
+										0x8f,
+										0xd0,
+										0xb4,
+										0xae,
+										0x2a,
+										0x30,
+										0x23,
+										0x92,
+										0x00,
+										0x7c,
+										0x82,
+										0x80,
+										0x00,
+										0x88,
+										0xd6,
+										0x65,
+										0xfa,
+										0xdd,
+										0xf4,
+										0x80,
+										0xfa,
+										0xe1,
+										0xa7,
+										0x44,
+										0x09,
+										0xea,
+										0xdd,
+									},
+									"field": nil,
 									"spans": []dataMap{},
 								},
 							},