update InitMetricReader to avoid returning server

[codeboten]

A follow up to #7871, the InitMetricReader method could be refactored to clean up the args and return values. See the comment:

// server returned here only happens when a pull based metric
// reader is configured, this could be refactored to pass in a
// func to add the server to the list of servers. another thing that would
// be nice is not to have to pass down the asyncErrorChannel

Originally posted by @mx-psi in #7871 (comment)