[sr-dev] [kamailio/kamailio] xhttp_prom module with custom tags (Issue #4021)

### Description

In Prometheus there is the possibility to tag values with tags. Those tags can then be used provide context to those values, for example to filter them.

### Expected behavior

It would be great if there was a way to provide custom tags to the metrics. These would essentially just be a static string between the name of the metric and its value.

Example: `system_load15{host="wurst23.dev.sipgate.net",hostname_short="wurst32"} 0.4`

This could probably be achieved by having an `xhttp_prom_ending` parameter similar to the `xhttp_prom_beginning` as we essentially just want to append a static string right after the metric name.

#### Actual observed behavior `kamailio_sip_lb_de_core_rcv_replies_1xx_invite 1773 1731419851663`