Add defaults processor to set default field values (#7370)

plugins/processors/defaults/README.md

@@ -0,0 +1,42 @@
+# Defaults Processor
+
+The *Defaults* processor allows you to ensure certain fields will always exist with a specified default value on your metric(s).
+
+There are three cases where this processor will insert a configured default field.
+
+1. The field is nil on the incoming metric
+1. The field is not nil, but its value is an empty string.
+1. The field is not nil, but its value is a string of one or more empty spaces.
+    
+### Configuration
+```toml
+## Set default fields on your metric(s) when they are nil or empty
+[[processors.defaults]]
+
+## This table determines what fields will be inserted in your metric(s)
+  [processors.defaults.fields]
+    field_1 = "bar"
+    time_idle = 0
+    is_error = true
+```
+
+### Example
+Ensure a _status\_code_ field with _N/A_ is inserted in the metric when one it's not set in the metric be default:
+
+```toml
+[[processors.defaults]]
+  [processors.defaults.fields]
+    status_code = "N/A"
+```
+
+```diff
+- lb,http_method=GET cache_status=HIT,latency=230
++ lb,http_method=GET cache_status=HIT,latency=230,status_code="N/A"
+```
+
+Ensure an empty string gets replaced by a default:
+
+```diff
+- lb,http_method=GET cache_status=HIT,latency=230,status_code=""
++ lb,http_method=GET cache_status=HIT,latency=230,status_code="N/A"
+```