Add to MetricRouter README

internal/metricRouter/README.md

@@ -51,6 +51,11 @@ The CCMetric router sits in between the collectors and the sinks and can be used
     ],
     "rename_metrics" : {
         "metric_12345" : "mymetric"
+    },
+    "normalize_units" : true,
+    "change_unit_prefix" {
+      "mem_used" : "G",
+      "mem_total" : "G"
     }
 }
 ```
@@ -170,6 +175,13 @@ This option takes a list of evaluable conditions and performs them one after the
 The first line is comparable with the example in `drop_metrics`, it drops all metrics starting with `drop_metric_` and ending with a number. The second line drops all metrics of the first hardware thread (**not** recommended)
 
 
+## The `normalize_units` option
+The cc-metric-collector tries to read the data from the system as it is reported. If available, it tries to read the metric unit from the system as well (e.g. from `/proc/meminfo`). The problem is that, depending on the source, the metric units are named differently. Just think about `byte`, `Byte`, `B`, `bytes`, ...
+The [cc-units](https://github.com/ClusterCockpit/cc-units) package provides us a normalization option to use the same metric unit name for all metrics. It this option is set to true, all `unit` meta tags are normalized.
+
+## The `change_unit_prefix` section
+It is often the case that metrics are reported by the system using a rather outdated unit prefix (like `/proc/meminfo` still uses kByte despite current memory sizes are in the GByte range). If you want to change the prefix of a unit, you can do that with the help of [cc-units](https://github.com/ClusterCockpit/cc-units). The setting works on the metric name and requires the new prefix for the metric. The cc-units package determines the scaling factor.
+
 # Aggregate metric values of the current interval with the `interval_aggregates` option
 
 **Note:** `interval_aggregates` works only if `num_cache_intervals` > 0