* add the ability to disable the gRPC API handler
(POLICIES_GRPC_DISABLED)
* add the ability to disable the Events API handler
(POLICIES_EVENTS_DISABLED)
* add metrics
* add support for specifying rego files via the environment varirable
POLICIES_ENGINE_FILES
* for file paths specified in yaml or in POLICIES_ENGINE_FILES, support
'config:' and 'data:' path prefixes
* fix typos in the documentation, and try to make it more clear
* add metrics to the documentation
* add a section for testing in the documentation
* introduces a new top-level package pkg/metrics/ with utilities for
metrics that are backported from the groupware branch, with unit
tests