[bug] PR Time to Merge should use average, not greatest

When looking at larger intervals of time, Kubernetes PR-Time-To-Merge uses this calculation:

``` greatest(percentile_disc(0.5) within group (order by open_to_lgtm asc), 0) as m_o2l_a,```

This is wrong; it leads to having a greater value the larger your time interval is, which is deceptive. For aggregating a median, there's two reasonable possibilities: a median of medians, or an average of medians. In practice, those two values are rarely that divergent, and an average is faster to calculate.

Therefore it should be:

```avg(percentile_disc(0.5) within group (order by open_to_lgtm asc), 0) as m_o2l_a,```

This mistake exists across all cacluations in this view.

Assigning to myself, will submit a PR later.

/assign