Regex: don't emit empty tags and fields when pattern doesn't match and result_key specified

[44px]

Fixed behavior when pattern doesn't match:
a) if no result_key specified => plugin doesn't alter existing tag/field
b) if result_key specified => plugin doesn't create tag/field with result_key name (resolves #4374)

Test telegraf.conf

[agent]
  interval="1s"
  flush_interval="1s"

[[inputs.exec]]
  timeout = "1s"
  data_format = "influx"
  commands = [
    "echo 'deal,computer_name=hosta message=\"stuff\" 1530654676316265790'",
    "echo 'deal,computer_name=hostb message=\"stuff\" 1530654676316265790'",
  ]

[[processors.regex]]
  [[processors.regex.tags]]
    key = "computer_name"
    pattern = "^(.*?)a$"
    replacement = "${1}"
    result_key = "server_name"
  [[processors.regex.tags]]
    key = "computer_name"
    pattern = "^(.*?)b$"
    replacement = "${1}"
    result_key = "server_name"

[[outputs.file]]
  files = ["stdout"]

Output

deal,computer_name=hostb,host=44px-notebook.local,server_name=host message="stuff" 1530654676000000000
deal,computer_name=hosta,host=44px-notebook.local,server_name=host message="stuff" 1530654676000000000

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.