Add time to first meaningful response to pull request metrics

We need a "time to first meaningful response" metric. The current plan for this is to add code to the pull request metrics scripts which will:

1. Pull the comments for each pull request for the week being parsed
2. Look for the first comment from a staff member of MariaDB Corporation or Foundation (maybe using a list of members to MariaDB GitHub?), opening user and comment user cannot be the same person.
3. Work out the time difference between open and first comment that meets clause 2
4. Average this over the week.

Any that do not have a first meaningful response will not count towards the time, but will be logged using a counter, giving a second metric of number of pull requests with no meaningful response.

Code for this is in the pull-requests directory of https://github.com/MariaDB/metrics