Thread specific summary counters

[kerolasa]

This pull request has three minor changes that are so trivial they hopefully do not need much discussion.

Main purpose of the change set is to add -threads=false command-line option support, that will make the thread specific exports to be summary counters. Level of detail is just too much in cases such as a company I work for, where we have lots of threads and lots of installations. That leads to prometheus server to reserve a lot of memory to keep track of these metrics, that are always summarised when visualised. Notice that the default per thread counting is kept as-is, new summary output is only for people who really want to merge the metrics.

[pgporada]

Gotcha, here's the old (default) behavior:

root@46dbabba90c5:/golang/unbound-pr-thread# curl -s localhost:9167/metrics | grep -v '#' | grep thread
go_threads 14
unbound_cache_hits_total{thread="0"} 3
unbound_cache_misses_total{thread="0"} 0
unbound_prefetches_total{thread="0"} 0
unbound_queries_total{thread="0"} 3
unbound_recursive_replies_total{thread="0"} 0
unbound_request_list_current_all{thread="0"} 0
unbound_request_list_current_user{thread="0"} 0
unbound_request_list_exceeded_total{thread="0"} 0
unbound_request_list_overwritten_total{thread="0"} 0

and behavior with the new flag

root@46dbabba90c5:/golang/unbound-pr-thread# curl -s localhost:9167/metrics | grep -v '#' | grep thread
go_threads 8
unbound_cache_hits_total{thread="sum"} 3
unbound_cache_misses_total{thread="sum"} 0
unbound_prefetches_total{thread="sum"} 0
unbound_queries_total{thread="sum"} 3
unbound_recursive_replies_total{thread="sum"} 0
unbound_request_list_current_all{thread="sum"} 0
unbound_request_list_current_user{thread="sum"} 0
unbound_request_list_exceeded_total{thread="sum"} 0
unbound_request_list_overwritten_total{thread="sum"} 0