Metric collection for Bazel issues

Quick and dirty tooling for metrics collection.

Usage:

1. Create a secrets.json file with two fields

{
  "client_id" : "...",
  "client_secret" : "..."
}

(get those from https://github.com/settings/developers)

1. issue-stats.py update will populate some all-*.json files with API responses from GitHub
2. Run various queries over those with issue-stats.py report and issue-stats.py garden
3. Feel free to modify those - I hope code is self-explanatory.