GitHub: add information about who closes and approves the PR

[canasdiaz]

Many review processes performed with GitHub depend on a team cloning a changeset, passing some tests and merging in a different commit. That is not visible with the standard GitHub tools, as the PRs are marked as closed without merged, while in reality the code is merged.

Some examples of this can be found in the Angular and Bazel communities:

• https://github.com/angular/angular/pull/48010
• https://github.com/bazelbuild/bazel/pull/15794

It is not possible to measure this with the standard metrics, but offering more information in the GitHub indexes may allow us to do it. What we need to display this metrics in our dashboards is to have extra information in the indexes where we have a document per PR. It can be the github_issues index, the github_pull_requests or both.

The extra information we need is:

• who closes the PR
• who approves the PR: this will be a multi-valued field
• who is pending to approve the PR: this will be a multi-valued field

This information is currently available in the github_events index, but with the current dashboard it is not possible to concatenate queries. We need this data in a single document.