docs: describe exported metrics

Also extend docs for new portal metrics.

README.md

@@ -3,6 +3,20 @@
 This is a [Prometheus](https://prometheus.io/) exporter for the
 [Cambium cnMaestro Cloud controller](https://cloud.cambiumnetworks.com).
 
+Currently, it exports the following metrics:
+
+- AP group status:
+  - number of devices (APs), config sync status, client count
+  - per AP:
+    - up- and downtime
+    - per radio:
+      - channel, channel width, power, quality, transfer rate
+- Guest access portals:
+  - number of active sessions per portal and per AP
+
+(This list might be outdated. The authoritative list is defined in the
+sources code, see [`collector.go`](./exporter/collector.go)).
+
 ## Installation
 
 ### Pre-built binary releases
@@ -55,24 +69,84 @@ corresponding metrics endpoints.
 
 Add a scrape config to your Prometheus configuration and reload Prometheus.
 
+This template sets up Prometheus to scrape two AP groups ("Default" and
+"another-group") and one guest access portal ("VisitorPortal"). Omit one
+or the other, if you don't use the portal feature, for example.
+
+In general, you should only need to fill in the `static_configs.targets`
+array:
+
 ```yml
 scrape_configs:
-  - job_name: cambium
+  - job_name: cambium_aps
+    static_configs:
+      - targets:
+        # Select the AP groups you care about.
+        # Visit http://localhost:9836 to view a list of available AP groups.
+        - Default
+        - another-group
     relabel_configs:
-      - source_labels: [__address__]
+      - # convert "NAME" to "/apgroups/NAME/metrics"
+        source_labels: [__address__]
         regex:         (.+)
         target_label:  __metrics_path__
         replacement:   /apgroups/$1/metrics
+      - # override "instance" label with address, i.e. the target name
+        source_labels: [__address__]
+        target_label: instance
+      - # overrride address (point to the exporter's real hostname:port)
+        target_label: __address__
+        replacement: 127.0.0.1:9836
+
+  - job_name: cambium_portals
+    static_configs:
+      - targets:
+        # Select the guest access portals you care about.
+        # Visit http://localhost:9836 to view a list of available portals.
+        - VisitorPortal
+    relabel_configs: # similar to the above
+      - source_labels: [__address__]
+        regex:         (.+)
+        target_label:  __metrics_path__
+        replacement:   /portals/$1/metrics
       - source_labels: [__address__]
         target_label: instance
       - target_label: __address__
-        replacement: 127.0.0.1:9836 # The exporter's real hostname:port
+        replacement: 127.0.0.1:9836
+```
+
+<details><summary>Using a combined jobs for AP groups and guest portals</summary>
+
+You can also combine the scrape config above. In this case, the naming of
+`static_configs.targets` changes, and you need to prefix either `apgroups/`
+or `portals/` to the target name.
+
+```yaml
+scrape_configs:
+  - job_name: cambium
     static_configs:
-      - targets: # select the AP groups you care about
-        - Default
-        - another-group
+      - targets:
+        # Select the AP groups and portals you care about.
+        # Visit http://localhost:9836 to view a list of available entries.
+        - apgroups/Default
+        - apgroups/another-group
+        - portals/VisitorPortal
+    relabel_configs:
+      - # convert "TYPE/NAME" to "/TYPE/NAME/metrics"
+        source_labels: [__address__]
+        regex:         (.+)
+        target_label:  __metrics_path__
+        replacement:   /$1/metrics
+      - # override "instance" label with address, i.e. the target name
+        source_labels: [__address__]
+        target_label: instance
+      - # overrride address (point to the exporter's real hostname:port)
+        target_label: __address__
+        replacement: 127.0.0.1:9836
 ```
 
+</summary>
+
 ## License
 
 This exporter is available as open soure under the terms of the