I want to set http_metrics mode using eventgen controller[FEATURE/IMPROVEMENT]

[Yangxulight]

Is your feature request related to a problem? Please describe.
I want to use "metric_httpevent" Outputmode under eventgen controller-server mode. But the setup api always overwrite outputmode to "httpevent".

Describe the solution you'd like
Read the outputmode from post body. We can give it an default value as "httpevent". User can give it a valid value from post body. See code details here : https://github.com/splunk/eventgen/blob/develop/splunk_eventgen/eventgen_api_server/eventgen_server_api.py, line 541.

[arctan5x]

#340

This PR addresses. All you need to do is to pass in "outputMode" key with metric_httpevent

[Rhutwi]

i am getting an error if i use output Mode as metric_httpevent in eventgen.conf.
do we have to define httpeventservers beforehand if we want eventgen data for metrics index