Add valuecounter aggregator plugin (#3523)

plugins/aggregators/valuecounter/README.md

@@ -0,0 +1,73 @@
+# ValueCounter Aggregator Plugin
+
+The valuecounter plugin counts the occurrence of values in fields and emits the
+counter once every 'period' seconds.
+
+A use case for the valuecounter plugin is when you are processing a HTTP access
+log (with the logparser input) and want to count the HTTP status codes.
+
+The fields which will be counted must be configured with the `fields`
+configuration directive. When no `fields` is provided the plugin will not count
+any fields. The results are emitted in fields in the format:
+`originalfieldname_fieldvalue = count`.
+
+Valuecounter only works on fields of the type int, bool or string. Float fields
+are being dropped to prevent the creating of too many fields.
+
+### Configuration:
+
+```toml
+[[aggregators.valuecounter]]
+  ## General Aggregator Arguments:
+  ## The period on which to flush & clear the aggregator.
+  period = "30s"
+  ## If true, the original metric will be dropped by the
+  ## aggregator and will not get sent to the output plugins.
+  drop_original = false
+  ## The fields for which the values will be counted
+  fields = ["status"]
+```
+
+### Measurements & Fields:
+
+- measurement1
+    - field_value1
+    - field_value2
+
+### Tags:
+
+No tags are applied by this aggregator.
+
+### Example Output:
+
+Example for parsing a HTTP access log.
+
+telegraf.conf:
+```
+[[inputs.logparser]]
+  files = ["/tmp/tst.log"]
+  [inputs.logparser.grok]
+    patterns = ['%{DATA:url:tag} %{NUMBER:response:string}']
+    measurement = "access"
+
+[[aggregators.valuecounter]]
+  namepass = ["access"]
+  fields = ["response"]
+```
+
+/tmp/tst.log
+```
+/some/path 200
+/some/path 401
+/some/path 200
+```
+
+```
+$ telegraf --config telegraf.conf --quiet
+
+access,url=/some/path,path=/tmp/tst.log,host=localhost.localdomain response="200" 1511948755991487011
+access,url=/some/path,path=/tmp/tst.log,host=localhost.localdomain response="401" 1511948755991522282
+access,url=/some/path,path=/tmp/tst.log,host=localhost.localdomain response="200" 1511948755991531697
+
+access,path=/tmp/tst.log,host=localhost.localdomain,url=/some/path response_200=2i,response_401=1i 1511948761000000000
+```