[sr-dev] [kamailio/kamailio] Active dialog stats stays static on a passive kamailio node that replicates dialogs via DMQ (#1692)

[Joel Serrano]

### Description

In a setup with 2 kamailios in active/passive with a virtual IP that failovers between them:

sbc01 (active) is handling calls.
sbc02 (passive) is started, and does an initial dialog sync via DMQ

```
root at sbc01:~# kamctl rpc stats.get_statistics all | grep dialog
    "dialog:active_dialogs = 39",
    "dialog:early_dialogs = 2",
    "dialog:expired_dialogs = 0",
    "dialog:failed_dialogs = 583",
    "dialog:processed_dialogs = 2527",
root at sbc01:~#
```

```
root at sbc02:/etc/kamailio# kamctl rpc stats.get_statistics all | grep dialog
    "dialog:active_dialogs = 39",
    "dialog:early_dialogs = 0",
    "dialog:expired_dialogs = 0",
    "dialog:failed_dialogs = 0",
    "dialog:processed_dialogs = 0",
root at sbc02:/etc/kamailio#
```

>From that point onwards, as long as sbc01 stays as active, sbc02 stays as passive, and no restarts occur anywhere, the values for `dialog` metrics in sbc02 will not change and remain as they are, in the previous example, active_dialogs would remain as 39.

### Troubleshooting

This was discovered during some tests from #1591

#### Reproduction

Setup 2 kamailio instances that replicate dialogs via DMQ.

1- On Kamailio1 put some traffic to have some active dialogs (must call `dlg_manage()` in routing script for these test calls)
2- Start Kamailio2
3- Compare output of: `kamcmd stats.get_statistics dialog:` on both nodes.
4- Alter the amount of active dialogs on Kamailio1 (either end some or create new ones).
5- Repeat step 3.

Kamailio1 --> Step5 will have different values from Step3.
Kamailio2 --> Step5 will have exactly the same values from Step3.


### Possible Solutions

`dialog:active_dialogs` should remain as 0 on Kamailio2, as technically there are no dialogs being handled by that server, it only has information of replicated dialogs and those stats are available in the `dlg.stats_active` metric.


### Additional Information

  * **Kamailio Version** - output of `kamailio -v`

```
# kamailio -V
version: kamailio 5.2.0-pre1 (x86_64/linux) 2ecf60-dirty
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, 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
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: 2ecf60 -dirty
compiled with gcc 6.3.0
```

* **Operating System**:

<!--
Details about the operating system, the type: Linux (e.g.,: Debian 8.4, Ubuntu 16.04, CentOS 7.1, ...), MacOS, xBSD, Solaris, ...;
Kernel details (output of `uname -a`)
-->

```
root at sbc02:~# lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 9.5 (stretch)
Release:	9.5
Codename:	stretch
root at sbc02:~# uname -a
Linux sbc02 4.9.0-8-amd64 #1 SMP Debian 4.9.110-3+deb9u6 (2018-10-08) x86_64 GNU/Linux
root at sbc02:~#
```


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