[metrics.latency] Add a new value type LatencyValue

[manugarg]

Changes:

• Remove AddInt64 and AddFloat64 from the Value interface. This lets us clean up all the implementations.
• Introduce a new interface type LatencyValue that supports the Values interface and AddFloat64(). Only Float and Distribution types need to meet this interface. As the name says, this interface will be used for latency values.
• Remove Inc and IncBy from the NumValue interface. Only "Int" type needs to support these methods.
• Add CloneDist method to Distribution type and use that instead of generic Clone() method that returns a Value.