Added support to auto discovery with SRV DNS registry (#60)

* Added support to auto discovery witih SRV

The yaml configuration has been modified to suport srv parameters.
Now the name is taken form the router identity to allow dynamic
discovery of devices based on SRV registry.

* Corrected format

* feat(collector): Added support to use custom DNS

* feat(collector): get default dns server from resolv.conf

* Remove getIdentity on the static config devices

README.md

@@ -53,6 +53,19 @@ devices:
     port: 8999
     user: prometheus2
     password: password_to_second_router
+  - name: routers_srv_dns
+    srv:
+      record: _mikrotik._udp.example.com
+    user: prometheus
+    password: password_to_all_dns_routers
+  - name: routers_srv_custom_dns
+    srv:
+      record: _mikrotik2._udp.example.com
+      dns:
+        address: 1.1.1.1
+        port: 53
+    user: prometheus
+    password: password_to_all_dns_routers
 
 features:
   bgp: true
@@ -64,6 +77,11 @@ features:
   optics: true
 ```
 
+If you add a devices with the `srv` parameter instead of `address` the exporter will perform a DNS query
+to obtain the SRV record and discover the devices dynamically. Also, you can specify a DNS server to use
+on the query.
+
+
 ###### example output
 
 ```