Prometheus Data Source for Apache Spark

This library is still work-in-progress. The main remaining TODO before GA are:

Create unit tests and end-to-end tests (via a mock HTTP server)
Polish the interface
- Automatically infer start and end time via query predicates
- Allow reading endpoint from a session wide config
Clean up the code (especially the JSON parsing part)
Error handling

Once polished, I will move this over to Databricks' official repository.