Add ability to set measurement from matched text in grok parser (#4433)

plugins/inputs/file/dev/docker-compose.yml

@@ -6,7 +6,7 @@ services:
       volumes:
         - ./telegraf.conf:/telegraf.conf
         - ../../../../telegraf:/telegraf
-        - ./json_a.log:/var/log/test.log
+        - ./dev/json_a.log:/var/log/test.log
       entrypoint:
         - /telegraf
         - --config

plugins/inputs/file/dev/json_a.log

@@ -1,14 +0,0 @@
-{
-"parent": {
-	"child": 3.0,
-	"ignored_child": "hi"
-},
-"ignored_null": null,
-"integer": 4,
-"list": [3, 4],
-"ignored_parent": {
-	"another_ignored_null": null,
-	"ignored_string": "hello, world!"
-},
-"another_list": [4]
-}

plugins/inputs/file/dev/testfiles/grok_a.log

@@ -0,0 +1,2 @@
+127.0.0.1 user-identifier frank [10/Oct/2000:13:55:36 -0700] "GET /apache_pb.gif HTTP/1.0" 200 2326
+128.0.0.1 user-identifier tony [10/Oct/2000:13:55:36 -0800] "GET /apache_pb.gif HTTP/1.0" 300 45

plugins/inputs/file/dev/testfiles/json_a.log

@@ -0,0 +1,14 @@
+{
+	"parent": {
+		"child": 3.0,
+		"ignored_child": "hi"
+	},
+	"ignored_null": null,
+	"integer": 4,
+	"list": [3, 4],
+	"ignored_parent": {
+		"another_ignored_null": null,
+		"ignored_string": "hello, world!"
+	},
+	"another_list": [4]
+}