WIP: new metrics emitter and histogram strategy

[Groxx]

Proof of concept on display.

This is an attempt to move away from our opaque and broadly-disliked metrics system, and towards two major changes:

• use structs to show what tags are available roughly at all time, to make our metrics-emitting easier to understand and safer to change (new fields == build failures until fixed)
• emit histograms rather than timers (this is just step 1 of like 1000+, to show the core tactic)

Once this runs in prod for a bit, so I have more data to play with, I'll build alerts and dashboards based on the new data, and we can check the runtime cost of these new metrics. I've selected moderately-used and rather-expensive (high cardinality, high number of calls) metrics for this first one, to try to give us a realistic sample.