Add "value" output data_format

[Paxa]

This PR add feature to output data "as is"

For example:

[[outputs.file]]
  files = ["stdout", "/tmp/metrics.out"]
  data_format = "value"

Will just print as received (value field)

User cases:

• Kafka mirror
• Process logs and saving local copy (for debugging or archiving)
• Forwarding data without modifications

It's just a prof of concept (i'm new to go lang), if you guys like this idea I will add test and documentation.

Btw will it be useful to send any fields other then value? (hard coded now)

[danielnelson]

I'm not sure about this serializer because the value format is so basic, and so the output would be very lossy.

The value parser is useful for interop with services that cannot be changed, but I'm not sure it is good for debugging or archiving when there are better choices. Wouldn't you rather have output in a format at least with a timestamp?

[Paxa]

I'm experimenting with data replication and data archiving. For example we have kafka with data points as json (event sourcing) and I wanna use telegraf to send this data somewhere else e.g. aws kinesis or other kafka server. So we can feed it to our application again later.

Standard serialiser will convert messages to something like:

kafka_consumer,host=MacBook-Pro.local value="{\"key\": \"value\"}" 1508428048076273000

But I wanna have just original message. Probably our situation is not a common use case, but may be it can be useful to forward data without extra formatting.

Wouldn't you rather have output in a format at least with a timestamp?

Most of apps already have timestamp in logs (e.g. rails, java, nodejs, telegraf)

[danielnelson]

Makes sense, could be useful for passthrough without paying for any validation/parsing. I would just leave it hardcoded to the value field.

[danielnelson]

I think this will print <nil>, if there is no value field I think it should return []byte("").

[danielnelson]

@Paxa Are you still working on this?

[Paxa]

No, I already don't need it

[danielnelson]

Okay, I'm going to close, perhaps we will add something like this in the future, but for now there are plenty of other things more pressing.