How do you measure your development team?

As a newly minted engineering manager, I'm curious how others measure their teams. I'm not particularly interested in trying to understand if particular developers are "good" (which I consider pretty much impossible to based on output) but if the team as a whole are performing well.

Some of the metrics I'm experimenting with at the moment are:

- Merged PRs in the last 30 days vs previous 30 day interval - Mean of how long work takes to get merged after being raised as a PR, compared to annual average - Mean of old PRs are when they get closed, also compared to annual average - Mean of average age of open PRs

I think that this gives me a good snapshot of how efficiently the team is currently performing and highlights areas to me where I need to drill down into more detail on what's going on.

I'm curious to see how others are doing this.