Proposal to simplify user-facing interfaces related to latency monitoring

[konjac]

For the current implementation, a common task for latency monitoring will require 4 user-facing interfaces

1. ILatencyContext which holds the context
2. ILatencyContextProvider which creates LatencyContext
3. ILatencyContextTokenIssuer which issues tokens for checkpoints/tags/measures.(BTW, There is an error in the document at README.md#latency-measurement. It is using ILatencyContext.GetCheckpointToken/GetMeasureToken. Both methods are actually defined in ILatencyContextTokenIssuer.)
4. ILatencyDataExporter which exports latency data.

So the monitored code should always inject 3 interfaces(No.2~4), which is quite wordy. What's more the implementation of LatencyContextTokenIssuer and LatencyContext create their own CheckpointTracker/MeasureTracker/TagCollection which is redundancy.

I would like to propose merge ILatencyContextTokenIssuer/ILatencyDataExporter into ILatencyContext. LatencyContext can hold references to ILatencyContextTokenIssuer/ILatencyDataExporter. Then it becomes the only entrance to issue token and export data.