Split NfsCollector in Nfs3Collector and Nfs4Collector (#28)

* Split NfsCollector in Nfs3Collector and Nfs4Collector

* Add documentation

collectors/nfs4Metric.md

@@ -0,0 +1,62 @@
+
+## `nfs4stat` collector
+
+```json
+  "nfs4stat": {
+    "nfsstat" : "/path/to/nfsstat",
+    "exclude_metrics": [
+      "nfs4_total"
+    ]
+  }
+```
+
+The `nfs4stat` collector reads data from `nfsstat` command and outputs a handful **node** metrics. If a metric is not required, it can be excluded from forwarding it to the sink. There is currently no possibility to get the metrics per mount point.
+
+
+Metrics:
+* `nfs4_total` 
+* `nfs4_null` 
+* `nfs4_read` 
+* `nfs4_write` 
+* `nfs4_commit` 
+* `nfs4_open` 
+* `nfs4_open_conf` 
+* `nfs4_open_noat` 
+* `nfs4_open_dgrd` 
+* `nfs4_close` 
+* `nfs4_setattr` 
+* `nfs4_fsinfo` 
+* `nfs4_renew` 
+* `nfs4_setclntid` 
+* `nfs4_confirm` 
+* `nfs4_lock` 
+* `nfs4_lockt` 
+* `nfs4_locku` 
+* `nfs4_access` 
+* `nfs4_getattr` 
+* `nfs4_lookup` 
+* `nfs4_lookup_root` 
+* `nfs4_remove` 
+* `nfs4_rename` 
+* `nfs4_link` 
+* `nfs4_symlink` 
+* `nfs4_create` 
+* `nfs4_pathconf` 
+* `nfs4_statfs` 
+* `nfs4_readlink` 
+* `nfs4_readdir` 
+* `nfs4_server_caps` 
+* `nfs4_delegreturn` 
+* `nfs4_getacl` 
+* `nfs4_setacl` 
+* `nfs4_rel_lkowner` 
+* `nfs4_exchange_id` 
+* `nfs4_create_session` 
+* `nfs4_destroy_session` 
+* `nfs4_sequence` 
+* `nfs4_get_lease_time` 
+* `nfs4_reclaim_comp` 
+* `nfs4_secinfo_no` 
+* `nfs4_bind_conn_to_ses` 
+
+