Proposal to simplify user-facing interfaces related to latency monitoring

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](https://github.com/dotnet/extensions/blob/main/src/Libraries/Microsoft.Extensions.Telemetry.Abstractions/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.