Update Metrics Transform Processor README (open-telemetry#2142)

processor/metricstransformprocessor/README.md

@@ -29,8 +29,12 @@ In addition to the above:
 
 - Operations can be applied to one or more metrics using a `strict` or `regexp`
  filter
-- Metrics can be updated in place or on an inserted copy
-- A set of matching metrics can be combined into a single metric
+- The `action` property allows metrics to be:
+ - Updated in-place (`update`)
+ - Copied and updates applied to the copy (`insert`)
+ - Combined into a newly inserted metric that is generated by combining all data
+ points from the set of matching metrics into a single metric (`combine`); the
+ original matching metrics are also removed
 - When renaming metrics, capturing groups from the `regexp` filter will be
  expanded
 - When adding or updating a label value, `{{version}}` will be replaced with
@@ -46,19 +50,28 @@ transformations or operations.
 ```yaml
 # transforms is a list of transformations with each element transforming a metric selected by metric name
 transforms:
+
+ # SPECIFY WHICH METRIC(S) TO MATCH
+
  # include specifies the metric name used to determine which metric(s) to operate on
  - include: <metric_name>
  # match_type specifies whether the include name should be used as a strict match or regexp match, default = strict
  match_type: {strict, regexp}
- # action specifies if the operations are performed on a metric in place, on an inserted clone, or if the matched metrics should be combined
+
+ # SPECIFY THE ACTION TO TAKE ON THE MATCHED METRIC(S)
+
+ # action specifies if the operations (specified below) are performed on metrics in place (update), on an inserted clone (insert), or on a new combined metric (combine)
  action: {update, insert, combine}
- # new_name specifies the updated name of the metric; if action is insert, new_name is required
+
+ # SPECIFY HOW TO TRANSFORM THE METRIC GENERATED AS A RESULT OF APPLYING THE ABOVE ACTION
+
+ # new_name specifies the updated name of the metric; if action is insert or combine, new_name is required
  new_name: <new_metric_name_inserted>
  # aggregation_type defines how combined data points will be aggregated; if action is combine, aggregation_type is required
  aggregation_type: {sum, mean, min, max}
  # submatch_case specifies the case that should be used when adding label values based on regexp submatches when performing a combine action; leave blank to use the submatch value as is
  submatch_case: {lower, upper}
- # operations contain a list of operations that will be performed on the selected metrics
+ # operations contain a list of operations that will be performed on the resulting metric(s)
  operations:
  # action defines the type of operation that will be performed, see examples below for more details
  - action: {add_label, update_label, delete_label_value, toggle_scalar_data_type, aggregate_labels, aggregate_label_values}
@@ -249,4 +262,4 @@ operations:
  match_type: regexp
  action: group
  group_resource_labels: {"resouce.type": "container", "source": "kubelet"}
-```
+```