afsocket-source: prometheus metrics inconsistencies

[alltilla]

Description of the problem

1. Sometimes the transport label is missing.
2. Setting the bind address as the address label is misleading.
3. Why don't we know the peer address in case of UDP?

Related code: https://github.com/syslog-ng/syslog-ng/blob/master/modules/afsocket/afsocket-source.c#L123

4. syslogng_socket_receive_dropped_packets_total fills the address label with the bind address, which is a bit confusing, we might want to rename it to bind_address or something similar.
5. transport can get the values udp, tcp, dgram, stream, which is a bit confusing, where sometimes only udp is used, sometimes only dgram, but both of them are meaningful in a way.

Proposed solution

Currently we follow the old stats instance name generation, but we can do better now if add some stats_cluster_label()s.

Example samples:

# syslog-ng-ctl stats -c /tmp/shared/syslog-ng.ctl prometheus|grep input_events_total
syslogng_input_events_total{transport="tcp",address="192.168.129.64",driver="afsocket",id="s_DEFAULT#4"} 4860923
syslogng_input_events_total{address="0.0.0.0",driver="afsocket",id="s_DEFAULT#1"} 0
syslogng_input_events_total{address="0.0.0.0",driver="afsocket",id="s_DEFAULT#2"} 117760
syslogng_input_events_total{transport="tcp",address="192.168.130.11",driver="afsocket",id="s_DEFAULT#4"} 305000
syslogng_input_events_total{address="0.0.0.0",driver="afsocket",id="s_DEFAULT#3"} 0
syslogng_input_events_total{address="0.0.0.0",driver="afsocket",id="s_DEFAULT#5"} 0
syslogng_input_events_total{address="0.0.0.0",driver="afsocket",id="s_DEFAULT#0"} 0
syslogng_input_events_total{transport="tcp",address="127.0.0.1",driver="afsocket",id="s_DEFAULT#4"} 588800
syslogng_input_events_total{transport="tcp",address="192.168.130.9",driver="afsocket",id="s_DEFAULT#4"} 298100