Set loggingService to logging-agent.target

It allows policymanager to manage stackdriver logging agents together.

BUG=b/192104059
TEST=stackdriver e2e test; presubmit
RELEASE_NOTE=None

Change-Id: I583547ff4cf996e957a3dba5f958f1ad45d92acc

cmd/monitor/monitor.go

diff --git a/cmd/monitor/monitor.go b/cmd/monitor/monitor.go
index d925e24..e941abf 100644
--- a/cmd/monitor/monitor.go
+++ b/cmd/monitor/monitor.go
@@ -61,7 +61,7 @@
 )
 
 var serviceMonitor = map[string]string{
-	"loggingService":    "stackdriver-logging.service",
+	"loggingService":    "logging-agent.target",
 	"monitoringService": "node-problem-detector.service",
 	"metricsService":    "crash-reporter.service",
 	"updateService":     "update-engine.service",