Add logon collector documentation

docs/collector.logon.md

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+# logon collector
+
+The logon collector exposes metrics detailing the active user logon sessions.
+
+|||
+-|-
+Metric name prefix  | `logon`
+Classes             | [`Win32_LogonSession`](https://docs.microsoft.com/en-us/windows/win32/cimwin32prov/win32-logonsession)
+Enabled by default? | No
+
+## Flags
+
+None
+
+## Metrics
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`wmi_logon_logon_type` | Number of active user logon sessions | gauge | status
+
+### Example metric
+Query the total number of interactive logon sessions
+```
+wmi_logon_logon_type{status="interactive"}
+```
+
+## Useful queries
+Query the total number of local and remote (I.E. Terminal Services) interactive sessions.
+```
+wmi_logon_logon_type{status=~"interactive|remoteinteractive"}
+```
+
+## Alerting examples
+_This collector does not yet have alerting examples, we would appreciate your help adding them!_