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>

.gitignore

@@ -10,10 +10,14 @@
 !*/
 !api
 
+# ignore
 .idea
 **/.DS_Store
 **/logs
 
+# ignore adhoc test code
+**/adhoc
+
 # gitlab ci
 .cache