pkg/server/server.go: add chunked data and served nar data metrics

[picnoir]

We introduce a /metrics prometheus endpoint exposing two metrics (on top of the golang runtime ones):

• nixcasync_chunkedDataGiB: total amount of uncompressed data sent to the chunker
• nixcasync_servedDataGiB: total amount of served NAR.

Part of #14

[picnoir]

Hmm, the put metrics are wrong. I don't have the credentials to convert this back to draft.

It's not ready for review yet.