Address Discovery TODO using singer get_standard_metadata

There's a TODO in the `discover()` function here: https://github.com/sisudata/tap-circle-ci/blob/master/tap_circle_ci/tap_circle_ci.py#L39 that could be addressed using a singer-python function.

At a minimum, you should pass through the schema and the key_properties: https://github.com/singer-io/singer-python/blob/master/singer/metadata.py#L25