Prometheus serializer config params not present in serializer config

[ibspoof]

Prometheus serializer enables several configuration parameters to change the output of the serializer. When using data_format=prometheus with http output telegraf config parsing errors on prometheus_* config params since they are not included in the config -> buildSerializer method.

Relevant telegraf.conf:

  data_format = "prometheus"

  prometheus_export_timestamp = true

System info:

Telegraf version 1.13

Steps to reproduce:

1. Enable outputs http w/ data_format= prometheus
2. Set prometheus_* config values in output config

[[outputs.http]]

  url = "http://127.0.0.1:3000/telegraf"

  data_format = "prometheus"
  
  prometheus_export_timestamp = true

  prometheus_sort_metrics = true

  prometheus_string_as_label = true

Expected behavior:

Startup will continue without error and prometheus data_format will serialize output

Actual behavior:

Error running agent: Error parsing ./telegraf.conf, line 17: field corresponding to `prometheus_sort_metrics' is not defined in http.HTTP

Additional info:

PR: #6823