Have the physical plan builder add metrics interceptors instead of
replacing the existing ones. It looks at the current setting, and
builds a list of interceptor classes/names out of it. Finally, the
builder appends the metrics collector interceptor to the list.

To test these changes I added an accessor for the underlying stream
config to QueryMetadata. The tests pull the configured interceptors
from the stream config and verify that they include both the client
and the metrics interceptors.

- Dont pass the stream configs around just for the sake of testing.
  Instead, factor out the last step of KafkaStreams construction into
  a factory interface that we can fake out in the test to intercept
  the args, which includes the streams config
- Split the client interceptor config before turning it into a list