filetime: add collector (#1639)

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

docs/collector.filetime.md

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+# filetime collector
+
+The filetime collector exposes modified timestamps of files in the filesystem.
+
+The collector
+
+|||
+-|-
+Metric name prefix  | `filetime`
+Enabled by default? | No
+
+## Flags
+
+### `--collectors.filetime.file-patterns`
+Comma-separated list of file patterns. Each pattern is a glob pattern that can contain `*`, `?`, and `**` (recursive).
+See https://github.com/bmatcuk/doublestar#patterns for an extended description of the pattern syntax.
+
+## Metrics
+
+Name | Description | Type | Labels
+-----|-------------|------|-------
+`windows_filetime_mtime_timestamp_seconds` | File modification time | gauge | `file`
+
+### Example metric
+
+```
+# HELP windows_filetime_mtime_timestamp_seconds File modification time
+# TYPE windows_filetime_mtime_timestamp_seconds gauge
+windows_filetime_mtime_timestamp_seconds{file="C:\\Users\\admin\\Desktop\\Dashboard.lnk"} 1.726434517e+09
+```
+
+## Useful queries
+_This collector does not yet have any useful queries added, we would appreciate your help adding them!_
+
+## Alerting examples
+_This collector does not yet have alerting examples, we would appreciate your help adding them!_