shallow-copy instead of (deep) Copy() in Merge()

Merge() must not modify self or other; shallow-copying is sufficient
to achieve that.

report/metadata_template.go

@@ -124,11 +124,14 @@ func (e MetadataTemplates) Copy() MetadataTemplates {
 // Merge merges two sets of MetadataTemplates so far just ignores based
 // on duplicate id key
 func (e MetadataTemplates) Merge(other MetadataTemplates) MetadataTemplates {
-	result := e.Copy()
+	if e == nil && other == nil {
+		return nil
+	}
+	result := make(MetadataTemplates, len(e))
+	for k, v := range e {
+		result[k] = v
+	}
 	for k, v := range other {
-		if result == nil {
-			result = MetadataTemplates{}
-		}
 		if existing, ok := result[k]; !ok || existing.Priority < v.Priority {
 			result[k] = v
 		}

report/metric_template.go

@@ -67,11 +67,14 @@ func (e MetricTemplates) Copy() MetricTemplates {
 // Merge merges two sets of MetricTemplates so far just ignores based
 // on duplicate id key
 func (e MetricTemplates) Merge(other MetricTemplates) MetricTemplates {
-	result := e.Copy()
+	if e == nil && other == nil {
+		return nil
+	}
+	result := make(MetricTemplates, len(e))
+	for k, v := range e {
+		result[k] = v
+	}
 	for k, v := range other {
-		if result == nil {
-			result = MetricTemplates{}
-		}
 		if existing, ok := result[k]; !ok || existing.Priority < v.Priority {
 			result[k] = v
 		}

report/table.go

@@ -154,11 +154,14 @@ func (t TableTemplates) Copy() TableTemplates {
 
 // Merge merges two sets of TableTemplates
 func (t TableTemplates) Merge(other TableTemplates) TableTemplates {
-	result := t.Copy()
+	if t == nil && other == nil {
+		return nil
+	}
+	result := make(TableTemplates, len(t))
+	for k, v := range t {
+		result[k] = v
+	}
 	for k, v := range other {
-		if result == nil {
-			result = TableTemplates{}
-		}
 		if existing, ok := result[k]; ok {
 			v = v.Merge(existing)
 		}

report/topology.go

@@ -175,7 +175,10 @@ func (n Nodes) Copy() Nodes {
 // Merge merges the other object into this one, and returns the result object.
 // The original is not modified.
 func (n Nodes) Merge(other Nodes) Nodes {
-	cp := n.Copy()
+	cp := make(Nodes, len(n))
+	for k, v := range n {
+		cp[k] = v
+	}
 	for k, v := range other {
 		if n, ok := cp[k]; ok { // don't overwrite
 			v = v.Merge(n)