RegisterCollectors register all the common static collector.

ServeProm return a server to serve prometheus metrics.

NamespaceEnvKey is the metric namespace key in environment.

SubsystemEnvKey is the metric subsystem key in environment.

TotalInFlightGauge used to collect total in flight number.

TotalReqCnt used to collect total request counter.

TotalReqDurSummary used to collect total request duration summaries.