"distrib" request handler metrics should only be tracked on pertinent handlers

SOLR-13979 in 8.4 added separate request handler metrics for distributed requests. However this was done for all request handlers, even though it's mainly SearchHandler (and maybe one or two others?) where a distributed request is even possible. I refer to this as "metrics pollution" and it's a bad thing. It's more weight per handler (latency load & memory), more weight for Solr metrics responses, and it's also suggestive that all registered handlers can have distributed requests when this is quite false, thus confusing people.