re-implement data usage crawler to be more efficient (#9075)

Implementation overview: 

https://gist.github.com/klauspost/1801c858d5e0df391114436fdad6987b

cmd/config/errors-utils.go

@@ -46,7 +46,7 @@ func (u Err) Clone() Err {
 	}
 }
 
-// Return the error message
+// Error returns the error message
 func (u Err) Error() string {
 	if u.detail == "" {
 		if u.msg != "" {

cmd/config/etcd/dns/etcd_dns.go

@@ -60,11 +60,7 @@ func (c *CoreDNS) List() (map[string][]SrvRecord, error) {
 			if record.Key == "" {
 				continue
 			}
-			if _, ok := srvRecords[record.Key]; ok {
-				srvRecords[record.Key] = append(srvRecords[record.Key], record)
-			} else {
-				srvRecords[record.Key] = []SrvRecord{record}
-			}
+			srvRecords[record.Key] = append(srvRecords[record.Key], record)
 		}
 	}
 	return srvRecords, nil