Expose "impressions" as OTel Metric

[toddbaert]

Impressions (basically evaluations) are a key metric for feature flag admins. We should capture this count by flag key, and variant (if reasonable).

[agardnerIT]

Can you provide some hints on how a contributor would begin tackling this or even pointing to the files on where to start?

[thisthat]

Hey @agardnerIT, I created a small PoC to showcase how these metrics can be exposed: thisthat#1

[beeme1mr]

That's great @thisthat!

[toddbaert]

@thisthat I've assigned this to you since you starting working on it, it seems. If you want to hand it off to someone else or you get too busy, just let me know I can re-assign.