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/beegfsstorageMetric.md

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+## `BeeGFS on Demand` collector
+This Collector is to collect BeeGFS on Demand (BeeOND) storage stats.
+
+```json
+  "beegfs_storage": {
+	"beegfs_path": "/usr/bin/beegfs-ctl",
+    "exclude_filesystem": [
+      "/mnt/ignore_me"
+    ],
+    "exclude_metrics": [     
+          "ack",
+		  "storInf",
+		  "unlnk"
+    ]
+  }
+```
+
+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 meta).
+For example beegfs metric `open`-> `beegfs_cstorage_`
+
+Note: BeeGFS FS offers many Metadata Information. Probably it makes sense to exlcude most of them. Nevertheless, these excluded metrics will be summed as `beegfs_cstorage_other`. 
+
+Available Metrics:
+
+* "sum"
+* "ack"
+* "sChDrct" 
+* "getFSize"
+* "sAttr"
+* "statfs"
+* "trunc"
+* "close"
+* "fsync"
+* "ops-rd"
+* "MiB-rd/s" 
+* "ops-wr"
+* "MiB-wr/s" 
+* "endbg" 
+* "hrtbeat"
+* "remNode"
+* "storInf"
+* "unlnk"
+
+
+The collector adds a `filesystem` tag to all metrics