Telegraf strings processor trim to length

[raffraffraff]

Feature Request

It should be possible to use the strings processor to trim values based on length. In some cases (eg: docker container IDs) a 12 character value is perfectly acceptable, but there is no way to do this with trim_right, as there isn't a specific delimiter that can be used

Proposal:

Add an additional optional flag to the configuration that performs simple substring using numerical position (eg: container_id[:12])

Current behavior:

You can only truncate on spaces or delimiters

Desired behavior:

You can truncate on position

Use case:

With docker in particular, there are image IDs and container IDs which are perfectly acceptable at a 12 character length (the default chosen by the docker cli). Across a large fleet this reduction could have a performance increase.

[danielnelson]

Seems useful, maybe we call it left (name inspired by postgres function):

[[processors.strings]]
  [[processors.strings.left]]
    field = "container_id"
    width = 12