Flag metrics as already registered in healthcheck

Signed-off-by: Eduardo J. Ortega U <5791035+ejortegau@users.noreply.github.com>

go/vt/discovery/healthcheck.go

@@ -886,6 +886,8 @@ func (hc *HealthCheckImpl) RegisterStats() {
 			"HealthcheckChecksum",
 			"crc32 checksum of the current healthcheck state",
 			hc.stateChecksum)
+
+		hc.statsRegistered = true
 	}
 }