Add prometheus serializer

[aixeshunter]

Feature Request

Opening a feature request kicks off a discussion.

Proposal:

Add a serializer that support Prometheus text exposition format

Current behavior:

No such serializer

Desired behavior:

[[outputs.file]]
  ## Files to write to, "stdout" is a specially handled file.
  files = ["stdout", "/tmp/metrics.out"]

  ## Data format to output.
  ## Each data format has its own unique set of configuration options, read
  ## more about them here:
  ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_OUTPUT.md
  data_format = "prometheus"

Use case: [Why is this important (helps with prioritizing requests)]

The telegraf was installed in VM, we hope the metrics will be stored in prometheus. And we have a component to get prometheus from vm serial port not http.

[danielnelson]

We have an open issue for this #4414.

I do have some concerns around streaming prometheus format in this way, as it won't behave identically to the prometheus output with respect to timestamps and multiple values. It may be preferable to send data_format = "influx" out of VM and convert to prometheus format outside using a second instance of Telegraf.

[aixeshunter]

@danielnelson Thanks for your answer.In my opinion, I installed the prometheus node-exporter in VM, telegraf which installed in vm got the metrics from node-exporter and output to file(vm virtual serial port).
Then, the external component that in hypervisor listened the serial port(linux socket) and got the metrics.
But in this way we have to maintain node-exporter in VM, and the telegraf just did the forwarding work.