### Description
We tried to use the `xhttp_prom` module, but our Prometheus couldn't read it,
because of metrics conflicting with Prometheus data model.
For us, the problem lies in the usrloc module. It exports the following metrics:
```
usrloc:location-contacts = 0
usrloc:location-expires = 0
usrloc:location-users = 0
usrloc:registered_users = 0
```
However, Prometheus allows only ascii characters and digits, as well as
underscores and colons.
See: https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels
The metrics names should not contain `-`, so they should get converted to `_`.
#### Reproduction
Setup Kamailio with `usrloc`, `xhttp` and `xhttp_prom` module and let
Prometheus query it. It will stumble upon the first entry of `usrloc` module
stats.
### Possible Solutions
Either the `usrloc` module could export its stats with the dash replaced by
underscores, or the xhttp_prom module could sanitize the output. The first one
would be a breaking change, I guess, making this a change for a major release.
### Additional Information
* **Kamailio Version** - output of `kamailio -v`
```
root@ifens5:/# kamailio -v
version: kamailio 5.3.8 (x86_64/linux)
flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE,
USE_MCAST, DNS_IP_HACK, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC,
DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE,
USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES,
TLS_PTHREAD_MUTEX_SHARED
ADAPTIVE_WAIT_LOOPS 1024, MAX_RECV_BUFFER_SIZE 262144, MAX_URI_SIZE 1024,
BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: unknown
compiled with gcc 7.5.0```
* **Operating System**:
```
Ubuntu 18.04
```
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https://github.com/kamailio/kamailio/issues/2644
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