* go_metrics: adds new runtime metrics
go_sched_latency_seconds - Distribution of the time goroutines have spent in the scheduler in a runnable state before actually running
go_mutex_wait_total_seconds - Approximate cumulative time goroutines have spent blocked on a sync.Mutex or sync.RWMutex
https://github.com/VictoriaMetrics/metrics/issues/54
* fixes data race
* add tests
* wip
* wip
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Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>