Beegfs collector (#50)

* added beegfs collectors to collectors/README.md

* added beegfs collectors and docs

* added new beegfs collectors to AvailableCollectors list

* Feedback implemented

* changed error type

* changed error to only return

* changed beegfs lookup path

* fixed typo in md files

Co-authored-by: Mehmet Soysal <mehmet.soysal@kit.edu>

collectors/beegfsmetaMetric.md

@@ -0,0 +1,75 @@
+## `BeeGFS on Demand` collector
+This Collector is to collect BeeGFS on Demand (BeeOND) metadata clientstats.
+
+```json
+  "beegfs_meta": {
+	"beegfs_path": "/usr/bin/beegfs-ctl",
+    "exclude_filesystem": [
+      "/mnt/ignore_me"
+    ],
+    "exclude_metrics": [     
+          "ack",
+          "entInf",
+          "fndOwn"
+    ]
+  }
+```
+
+The `BeeGFS On Demand (BeeOND)` collector uses the `beegfs-ctl` command to read performance metrics for
+BeeGFS filesystems.
+
+The reported filesystems can be filtered with the `exclude_filesystem` option
+in the configuration.
+
+The path to the `beegfs-ctl` command can be configured with the `beegfs_path` option
+in the configuration.
+
+When using the `exclude_metrics` option, the excluded metrics are summed as `other`.
+
+Important: The metrics listed below, are similar to the naming of BeeGFS. The Collector prefixes these with `beegfs_cstorage`(beegfs client storage).
+
+For example beegfs metric `open`-> `beegfs_cstorage_open`
+
+Available Metrics:
+
+* sum
+* ack
+* close
+* entInf
+* fndOwn
+* mkdir
+* create
+* rddir
+* refrEnt
+* mdsInf
+* rmdir
+* rmLnk
+* mvDirIns
+* mvFiIns
+* open
+* ren
+* sChDrct
+* sAttr
+* sDirPat
+* stat
+* statfs
+* trunc
+* symlnk
+* unlnk
+* lookLI
+* statLI
+* revalLI
+* openLI
+* createLI
+* hardlnk
+* flckAp
+* flckEn
+* flckRg
+* dirparent
+* listXA
+* getXA
+* rmXA
+* setXA
+* mirror
+
+The collector adds a `filesystem` tag to all metrics