WIP Integrate prometheus telemetry

This change introduces core metrics exposed via prometheus. The current metrics include request/response sizes and response timings. Labels are used to provide dimensions like http response codes and bucket names.

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The commits in this branch will be cleaned up in time and likely stashed to a single coherent change-set
/rfr @bernerdschaefer @grobie @juliusv @u-c-l 

looking for:
* more metrics to expose
* alternate approach for the integration
* general comments