Add merge aggregator (#6410)

plugins/aggregators/merge/README.md

@@ -0,0 +1,23 @@
+# Merge Aggregator
+
+Merge metrics together into a metric with multiple fields into the most memory
+and network transfer efficient form.
+
+Use this plugin when fields are split over multiple metrics, with the same
+measurement, tag set and timestamp.  By merging into a single metric they can
+be handled more efficiently by the output.
+
+### Configuration
+
+```toml
+[[aggregators.merge]]
+  # no configuration
+```
+
+### Example
+
+```diff
+- cpu,host=localhost usage_time=42 1567562620000000000
+- cpu,host=localhost idle_time=42 1567562620000000000
++ cpu,host=localhost idle_time=42,usage_time=42 1567562620000000000
+```