Use a random client id so multiple instances can connect simultaneously

https://stackoverflow.com/questions/59625907/how-to-handle-multiple-clients-with-same-id-in-mosquitto-mqtt

Author: infertux

TODO

@@ -1,3 +1,4 @@
+- FEATURE: provide binaries built with CI
 - BUGFIX: automatically reconnect to MQTT broker when connection lost
 - add program option --verbose instead of compile-time LOG_VERBOSE define
 - test program with either --mqtt XOR --prometheus but not both

src/mqtt.h

@@ -56,7 +56,7 @@ int start_mqtt_thread(void *config_ptr) {
 
   mosquitto_lib_init();
 
-  client = mosquitto_new("growatt-exporter", true, NULL);
+  client = mosquitto_new(NULL, true, NULL);
   if (!client) {
     PERROR("Cannot create mosquitto client instance");
     return EXIT_FAILURE;