collector: support custom poll intervals

Also simplify config context schema for SNMP since we only use a single
instance.

roles/collector/README.md

@@ -1,18 +1,19 @@
 Set up metric collection with prometheus and telegraf as the SNMP proxy.
 
-NetBox config context should contain the lists `prometheus_hosts` and `snmp_hosts` with job definitions. Each entry should define `name` and `nb_filter` user to query hosts from NetBox. For example:
+Each entry in `prometheus_config` should define `name`, `hosts` and optionally `interval`. As above, `hosts` is used as a query filter.
+
+For SNMP the properties `snmp_hosts` and optional `snmp_interval` should define respectively the NetBox query filter and poll interval.
+
+For example:
 
     {
-        "prometheus_hosts": [
+        "prometheus_config": [
             {
                 "name": "classroom",
-                "nb_filter": "role=desktop-computer status=active location=classroom"
+                "hosts": "role=desktop-computer status=active location=classroom",
+                "interval": 300
             }
         ],
-        "snmp_hosts": [
-            {
-                "name": "switch",
-                "nb_filter": "role=switch name__isw=sw- status=active status=staged status=planned"
-            }
-        ]
+        "snmp_hosts": "role=switch name__isw=sw- status=active status=staged status=planned",
+        "snmp_interval": 300
     }