Provide number of samples and number of variants endpoint.

[grosscol]

Is your feature request related to a specific question or information need?
Providing metrics for the volume of information that is backing the API. This is information that is currently provided by the UI by hard coding it. It is likely to be missed and requires a code difference when deploying with different backing data sets.

Describe the solution you'd like
An endpoint that caches the result of the tabulation for the entire lifetime of the application such that it will only count the number of samples and variants once.

Additional context
The UI currently requires the number of samples and variants to be hardcoded statgen/bravo_vue#4. This feature removes a configuration step for different data sets.

[grosscol]

Number of samples can't be determined from the processed data that is currently backing the API. That part of the issue has been pushed upstream to statgen/bravo_data_prep#9