The 'config' argument to Metric.constructor is not a defined type and is open-ended

The typescript type puts a limit on what fields to populate from this object, however the implementation just does Object.assign(this, ...) which is all sorts of fun for prototype pollution, and also makes #812 a bit of a challenge since basically anything goes.

In fact it was already previously being abused by summary.js to set the store, which is not an allowed value for that data structure.

I'm going to want to make this concrete to fix the labels() issue without making a giant footgun for everyone else. We won't touch a config field unless we already know the name. But it can be folded into the validation section which should keep things from getting long winded.

Start by examining Metric.constructor and the config type, then inspect summary.js where store is assigned through the config object. Trace the existing validation section and identify which config field names are supported. Done means unknown fields are not assigned, the labels() change remains possible, and the existing config behavior is covered by validation.