Add override processor (#3773)

This plugin can perform the standard metric modifications using override semantics.

plugins/processors/override/README.md

@@ -0,0 +1,30 @@
+# Override Processor Plugin
+
+The override processor plugin allows overriding all modifications that are
+supported by input plugins and aggregators:
+
+* name_override
+* name_prefix
+* name_suffix
+* tags
+
+All metrics passing through this processor will be modified accordingly. Values
+of *name_override*, *name_prefix*, *name_suffix* and already present *tags* with
+conflicting keys will be overwritten. Absent *tags* will be created.
+
+Use-case of this plugin encompass ensuring certain tags or naming conventions
+are adhered to irrespective of input plugin configurations, e.g. by
+`taginclude`.
+
+### Configuration:
+
+```toml
+# Add a global tag to all metrics
+[[processors.override]]
+  name_override = "new name_override"
+  name_prefix = "new name_prefix"
+  name_suffix = ":new name_suffix"
+  [processors.tags.add]
+    additional_tag = "tag_value"
+    existing_tag = "new tag_value"
+```