"Number of pull requests" data appears inaccurate/misleading

In the velocity reports, we report "The y-axis is the total **number of pull requests** and issues".

From the query, this is determined by the total amount of `PullRequestEvent`s: https://github.com/cncf/velocity/blob/8e1d1c189b65e2544fae7aec43c6381f9e4b4d82/BigQuery/velocity_cncf.sql#L19C18-L19C34.

A `PullRequestEvent` does not correlate 1:1 with "a PR" in a way a person would interpret a count of PRs, in my opinion. There are two reasonable approaches (merged PRs or opened PRs, strongly preferring merged PRs), neither of which this counts.

Per [docs](https://docs.github.com/en/rest/overview/github-event-types?apiVersion=2022-11-28#pullrequestevent) 'The action that was performed. Can be one of opened, edited, closed, reopened, assigned, unassigned, review_requested, review_request_removed, labeled, unlabeled, and synchronize.'. However, in practice I found this doesn't seem to be the case. Looking at a single day across github:

```
1916 reopened
168129 closed
193220 opened
```

Even without the other possible events, we at least appear to be double counting PRs?