Improve error handling of incorrect types in configuration

[adrianlzt]

Feature Request

Telegraf silenty ignore parameters with incorrectly defined types.

In some configurations, this could lead to an unxpected behaviour with a solution difficult to be spotted.

Proposal:

Telegraf should fail if a parameter is defined with the incorrect type.

Current behavior:

If tagpass/tagdrop/namepass/namedrop/fieldpass/fielddrop is defined using the incorrect type, it is ignored

Desired behavior:

Refuse to start if the parameter is found with the incorrect type.

Use case:

Example with tagpass, defined as an array of tables instead of a table.
In this case tagpass will be ignored.

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

[[inputs.kernel]]

[[processors.printer]]
  [[processors.printer.tagpass]]  # THIS should be a table
    host = ["hostname"]

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

If tagpass is not a table, there is not an else condition to return an error.

Same happens if we defined host as string.
In this case the host filter will be ignored.

[[processors.printer]]
  [processors.printer.tagpass]
    host = "hostname"   # THIS should be an array

[danielnelson]

This is closely related to #3444, I believe to resolve we need to make the move to the github.com/BurntSushi/toml library for parsing the configuration file.

[powersj]

Hi,

It appears that the examples using tag pass as a string and not a slice now correctly fail with #8910:

2022-08-08T18:51:18Z E! [telegraf] Error running agent: Error loading config file config.toml: error parsing printer, line 8:{105 127}: found unexpected format while parsing "tagpass", expecting string array/slice format on each entry

Closing as fixed, but if we have missed something, please try with the latest telegraf, and provide an example and logs.