chore(sentry): Sentry opt out banner. (#5672) (#5727)

Fixes DGRAPH-1689.

(cherry picked from commit eeb0dcb)

dgraph/cmd/alpha/run.go

@@ -555,6 +555,7 @@ func run() {
 		defer x.FlushSentry()
 		x.ConfigureSentryScope("alpha")
 		x.WrapPanics()
+		x.SentryOptOutNote()
 	}
 	bindall = Alpha.Conf.GetBool("bindall")
 

dgraph/cmd/zero/run.go

@@ -169,6 +169,7 @@ func run() {
 		defer x.FlushSentry()
 		x.ConfigureSentryScope("zero")
 		x.WrapPanics()
+		x.SentryOptOutNote()
 	}
 
 	x.PrintVersion()

x/sentry_integration.go

@@ -43,6 +43,14 @@ const (
 	dsnDevtest = "https://84c2ad450005436fa27d97ef72b52425@o318308.ingest.sentry.io/5208688"
 )
 
+// SentryOptOutNote - This is an opt out banner.
+func SentryOptOutNote() {
+	glog.Infof("This instance of Dgraph will send anonymous reports of panics back " +
+		"to Dgraph Labs via Sentry. No confidential information is sent. These reports " +
+		"help improve Dgraph. To opt-out, restart your instance with the --enable_sentry=false " +
+		"flag. For more info, see https://dgraph.io/docs/howto/#data-handling.")
+}
+
 // InitSentry initializes the sentry machinery.
 func InitSentry(ee bool) {
 	env = "prod-"