Add filecount input plugin (#4363)

plugins/inputs/filecount/README.md

@@ -0,0 +1,49 @@
+# filecount Input Plugin
+
+Counts files in directories that match certain criteria.
+
+### Configuration:
+
+```toml
+# Count files in a directory
+[[inputs.filecount]]
+  ## Directory to gather stats about.
+  directory = "/var/cache/apt/archives"
+
+  ## Only count files that match the name pattern. Defaults to "*".
+  name = "*.deb"
+
+  ## Count files in subdirectories. Defaults to true.
+  recursive = false
+
+  ## Only count regular files. Defaults to true.
+  regular_only = true
+
+  ## Only count files that are at least this size in bytes. If size is
+  ## a negative number, only count files that are smaller than the
+  ## absolute value of size. Defaults to 0.
+  size = 0
+
+  ## Only count files that have not been touched for at least this
+  ## duration. If mtime is negative, only count files that have been
+  ## touched in this duration. Defaults to "0s".
+  mtime = "0s"
+```
+
+### Measurements & Fields:
+
+- filecount
+    - count (int)
+
+### Tags:
+
+- All measurements have the following tags:
+    - directory (the directory path, as specified in the config)
+
+### Example Output:
+
+```
+$ telegraf --config /etc/telegraf/telegraf.conf --input-filter filecount --test
+> filecount,directory=/var/cache/apt,host=czernobog count=7i 1530034445000000000
+> filecount,directory=/tmp,host=czernobog count=17i 1530034445000000000
+```