Use enums to replace attribute's runtime prefix function

Instead of always dynamically creating a prefix function to determine
the prefix for Zipkin attributes, we use enumerations of the direction
and create strings at compile time. This is similar to the work that we did
on the Zipkin exporter

Author: Paulo Janotti

receiver/zipkin/trace_receiver.go

@@ -26,6 +26,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/http"
+	"strconv"
 	"strings"
 	"sync"
 
@@ -392,7 +393,7 @@ func nodeFromZipkinEndpoints(zs *zipkinmodel.SpanModel) *commonpb.Node {
 		node.ServiceInfo = &commonpb.ServiceInfo{
 			Name: lep.ServiceName,
 		}
-		node.Attributes = zipkinEndpointIntoAttributes(lep, node.Attributes, func(s string) string { return s })
+		node.Attributes = zipkinEndpointIntoAttributes(lep, node.Attributes, isLocalEndpoint)
 	}
 
 	// Retrieve and make use of the remote endpoint
@@ -404,30 +405,44 @@ func nodeFromZipkinEndpoints(zs *zipkinmodel.SpanModel) *commonpb.Node {
 		//      "zipkin.remoteEndpoint.port": "9000"
 		//      "zipkin.remoteEndpoint.serviceName": "backend",
 		// }
-		node.Attributes = zipkinEndpointIntoAttributes(rep, node.Attributes, func(s string) string {
-			return "zipkin.remoteEndpoint." + s
-		})
+		node.Attributes = zipkinEndpointIntoAttributes(rep, node.Attributes, isRemoteEndpoint)
 	}
 	return node
 }
 
+type zipkinDirection bool
+
+const (
+	isLocalEndpoint  zipkinDirection = true
+	isRemoteEndpoint zipkinDirection = false
+)
+
 var blankIP net.IP
 
-func zipkinEndpointIntoAttributes(ep *zipkinmodel.Endpoint, into map[string]string, prefixFunc func(string) string) map[string]string {
+func zipkinEndpointIntoAttributes(ep *zipkinmodel.Endpoint, into map[string]string, endpointType zipkinDirection) map[string]string {
 	if into == nil {
 		into = make(map[string]string)
 	}
+
+	var ipv4Key, ipv6Key, portKey, serviceNameKey string
+	if endpointType == isLocalEndpoint {
+		ipv4Key, ipv6Key = "ipv4", "ipv6"
+		portKey, serviceNameKey = "port", "serviceName"
+	} else {
+		ipv4Key, ipv6Key = "zipkin.remoteEndpoint.ipv4", "zipkin.remoteEndpoint.ipv6"
+		portKey, serviceNameKey = "zipkin.remoteEndpoint.port", "zipkin.remoteEndpoint.serviceName"
+	}
 	if ep.IPv4 != nil && !ep.IPv4.Equal(blankIP) {
-		into[prefixFunc("ipv4")] = ep.IPv4.String()
+		into[ipv4Key] = ep.IPv4.String()
 	}
 	if ep.IPv6 != nil && !ep.IPv6.Equal(blankIP) {
-		into[prefixFunc("ipv6")] = ep.IPv6.String()
+		into[ipv6Key] = ep.IPv6.String()
 	}
 	if ep.Port > 0 {
-		into[prefixFunc("port")] = fmt.Sprintf("%d", ep.Port)
+		into[portKey] = strconv.Itoa(int(ep.Port))
 	}
 	if serviceName := ep.ServiceName; serviceName != "" {
-		into[prefixFunc("serviceName")] = serviceName
+		into[serviceNameKey] = serviceName
 	}
 	return into
 }