* Replaced ClusterMetricSink's cluster name with an atomic.Value.
This should permit go-race tests to pass which seal and unseal
the core.
* Replace metric sink before unseal to avoid data races.
* Add token creation counters.
* Created a utility to change TTL to bucket name.
* Add counter covering token creation for response wrapping.
* Fix namespace label, with a new utility function.
* initial commit for prometheus and sys/metrics support
* Throw an error if prometheusRetentionTime is 0,add prometheus in devmode
* return when format=prometheus is used and prom is disable
* parse prometheus_retention_time from string instead of int
* Initialize config.Telemetry if nil
* address PR issues
* add sys/metrics framework.Path in a factory
* Apply requiredMountTable entries's MountConfig to existing core table
* address pr comments
* enable prometheus sink by default
* Move Metric-related code in a separate metricsutil helper
