Adds basic Prometheus support

[benhaan]

New /metrics endpoint that Prometheus can scrape. Metrics provided include response times, image transformation time, remote fetch error count, and the number of requests served from cache.

I'm still new to Go, so any input/feedback on the implementation is appreciated.

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[benhaan]

I signed it!

[googlebot]

CLAs look good, thanks!

[alexesDev]

@benhaan this is a good feature, but the branch not merged. Why?

[BrianGreenhill]

+1 for this feature 🙏

[willnorris]

Sorry for the long delay. Merged with a few minor changes.

[BrianGreenhill]

thank you @willnorris 🙏