Added doc for Scheduled Task collector

Signed-off-by: Rahman Mousavian <rahman.mousavian@oracle.com>

docs/collector.scheduled_task.md

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+# scheduled_task collector
+
+The scheduled_task collector exposes metrics about Windows Task Scheduler
+
+|||
+-|-
+Metric name prefix  | `scheduled_task`
+Data source         | OLE
+Enabled by default? | No
+
+## Flags
+
+### `--collector.scheduled_task.whitelist`
+
+If given, the path of the task needs to match the whitelist regexp in order for the corresponding metrics to be reported.
+
+### `--collector.scheduled_task.blacklist`
+
+If given, the path of the task needs to *not* match the blacklist regexp in order for the corresponding metrics to be reported.
+
+## Metrics
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`windows_scheduled_task_last_result` | The result that was returned the last time the registered task was run | gauge | task
+`windows_scheduled_task_missed_runs` | The number of times the registered task missed a scheduled run | gauge | task
+`windows_scheduled_task_state` | The current state of a scheduled task | gauge | task, state
+
+For the values of the `state` label, see below.
+
+### State
+
+A task can be in the following states:
+- `disabled`
+- `queued`
+- `ready`
+- `running`
+- `unknown`
+
+
+### Example metric
+
+```
+windows_scheduled_task_last_result{task="/Microsoft/Windows/Chkdsk/SyspartRepair"} 1
+windows_scheduled_task_missed_runs{task="/Microsoft/Windows/Chkdsk/SyspartRepair"} 0
+windows_scheduled_task_state{state="disabled",task="/Microsoft/Windows/Chkdsk/SyspartRepair"} 1
+windows_scheduled_task_state{state="queued",task="/Microsoft/Windows/Chkdsk/SyspartRepair"} 0
+windows_scheduled_task_state{state="ready",task="/Microsoft/Windows/Chkdsk/SyspartRepair"} 0
+windows_scheduled_task_state{state="running",task="/Microsoft/Windows/Chkdsk/SyspartRepair"} 0
+windows_scheduled_task_state{state="unknown",task="/Microsoft/Windows/Chkdsk/SyspartRepair"} 0
+```
+
+## Useful queries
+_This collector does not yet have any useful queries added, we would appreciate your help adding them!_
+
+## Alerting examples
+**prometheus.rules**
+```yaml
+  - alert: "WindowsScheduledTaskFailure"
+    expr: "windows_scheduled_task_last_result == 0"
+    for: "1d"
+    labels:
+      severity: "high"
+    annotations:
+      summary: "Scheduled Task Failed"
+      description: "Scheduled task '{{ $labels.task }}' failed for 1 day"
+```