Derived metrics (#57)

* Add time-based derivatived (e.g. bandwidth) to some collectors

* Add documentation

* Add comments

* Fix: Only compute rates with a valid previous state

* Only compute rates with a valid previous state

* Define const values for net/dev fields

* Set default config values

* Add comments

* Refactor: Consolidate data structures

* Refactor: Consolidate data structures

* Refactor: Avoid struct deep copy

* Refactor: Avoid redundant tag maps

* Refactor: Use int64 type for absolut values

Co-authored-by: Holger Obermaier <40787752+ho-ob@users.noreply.github.com>

collectors.json

@@ -12,6 +12,12 @@
             "proc_total"
         ]
     },
+    "netstat": {
+        "include_devices": [
+            "enp5s0"
+        ],
+        "send_derived_values": true
+    },
     "numastats": {},
     "nvidia": {},
     "tempstat": {