set exporter endpoint & sample rate in startup script

ext/central-controller-docker/main.sh

@@ -83,6 +83,8 @@ echo "{
         \"lowBandwidthMode\": ${ZT_LB_MODE:-$DEFAULT_LB_MODE},
         \"ssoRedirectURL\": \"${ZT_SSO_REDIRECT_URL}\",
         \"allowManagementFrom\": [\"127.0.0.1\", \"::1\", \"10.0.0.0/8\"],
+        \"exporterEndpoint\": \"${ZT_EXPORTER_ENDPOINT}\",
+        \"exporterSampleRate\": ${ZT_EXPORTER_SAMPLE_RATE:-0},
         ${REDIS}
     }
 }