Support metrics aggregation in Ray runner

[valiantljk]

Related Issue: https://github.com/ray-project/ray_beam_runner/issues/47

This PR implemented the metrics aggregation in the Ray runner. Supported metrics include:

1. user-defined element-count metrics, e.g., counter, gauge, distribution (min, max, avg, etc)
2. non-user metrics, e.g., start_bundle_msecs, total_msecs, etc

The PR also provides an example using user-defined metrics in word-count. To run the example:

python examples/word_count_metrics.py --input input.txt

With a random generated input.txt, the expected output should be like:

Ray Runner--------------->
number of empty lines:98
average word length:4.18
min word length: 1
max word length: 14
Direct Runner--------------->
number of empty lines:98
average word length:4.18
min word length: 1
max word length: 14

[pabloem]

this is really great. Thanks! : )