Add docs for topprocs collector

collectors/topprocsMetric.md

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+
+## `topprocs` collector
+
+```json
+  "topprocs": {
+    "num_procs": 5
+  }
+```
+
+The `topprocs` collector reads the TopX processes (sorted by CPU utilization, `ps -Ao comm --sort=-pcpu`). 
+
+In contrast to most other collectors, the metric value is a `string`.
+
+
+