implement k8s service discovery (#988)

* implement k8s service discovery

* simplify code

* use default namespace if not provided

* disable codecov bot comment

* ignore adhoc dir

* simplify building target in NewClient

* reformat code

* Fix filepath (#990)

* format code, and reorg imports (#991)

* add more unit test

Co-authored-by: anqiansong <anqiansong@gmail.com>

zrpc/internal/resolver/kube/targetparser.go

@@ -0,0 +1,45 @@
+package kube
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"google.golang.org/grpc/resolver"
+)
+
+const (
+	colon            = ":"
+	defaultNamespace = "default"
+)
+
+var emptyService Service
+
+type Service struct {
+	Namespace string
+	Name      string
+	Port      int
+}
+
+func ParseTarget(target resolver.Target) (Service, error) {
+	var service Service
+	service.Namespace = target.Authority
+	if len(service.Namespace) == 0 {
+		service.Namespace = defaultNamespace
+	}
+
+	segs := strings.SplitN(target.Endpoint, colon, 2)
+	if len(segs) < 2 {
+		return emptyService, fmt.Errorf("bad endpoint: %s", target.Endpoint)
+	}
+
+	service.Name = segs[0]
+	port, err := strconv.Atoi(segs[1])
+	if err != nil {
+		return emptyService, err
+	}
+
+	service.Port = port
+
+	return service, nil
+}