Show metrics in a (more) human-readable format

Ideally we could find a way to make the Flink metrics protos available to the Python client without toString, since toString should generally should be human-readable. The problem is that without a job server, there's nowhere to access job information like metrics outside of what Flink provides natively.
 
This issue was introduced here: https://github.com/apache/beam/pull/11369/files#diff-7fd541906a9e771c3312b75947e969daL176