7595 - Be able to add the body's content as field with http_response

[essobedo]

closes #7595

Required for all PRs:

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

[danielnelson]

I think we need to include a few checks before we can do this safely.

Make sure the body is valid utf-8. While technically a string field could contain any data, we want to have all strings from Telegraf to be valid utf-8. If the body is not valid utf-8, log an error and report body_read_error.

Add the ability to set a limit on the body size: response_body_max_size = "1KiB". Check the influxdb_listener for an example using http.MaxBytesReader. If it is exceeded report a body_read_error and log.

Let's also call the new option: response_body_field = "body". Default will be "" which means don't add.

[essobedo]

@danielnelson thx for your feedback. I did the corresponding changes however please note that regarding response_body_max_size:

1. I made it non optional with a default value of 32 Mo (it is big but we did not have any limit before this change)
2. I decided not to use MaxBytesReader because it seems to be for request body only such that you end up with an error message of type "http: request body too large" (as you can see here) which would be misleading I believe so I used io.LimitReader instead and made it read one byte above the limit to know if the limit has been reached.

WDYT of these changes?

[essobedo]

@danielnelson please note that the CI fails because of another test that doesn't seem to be related to those changes

[essobedo]

@danielnelson thx