Metrics should follow data-flow

[mariusae]

In the current implementation of metrics (#18), metrics are collected and aggregated from all tasks involved at the completion of a session.Run. This has a few drawbacks. First, all tasks may not be available at the end of session.Run (e.g., due to failed machines). Second, it ties stats collection to evaluation: we may in the future, persist task outputs to, e.g., s3, and clear task outputs from individual machines.

The solution to this is clear: we can flow Scopes along with task outputs; the executor can then aggregate them as the data flows.