This change adds support to configurable prometheus buckets

http_request_duration has fixed, hardcoded number of buckets with no possibility to tweak them
For cases when the most of the latencies are above 1ms, with only 4 available buckets there's no good insight on OPA's performance.

This implementation:
- adds the possibility for the buckets to be configurable in ```server.metrics.prom.http_request_duration_seconds.buckets``` key
- it's not a breaking change, if the buckets are not present in the configuration, the metric is configured with the existing values as a fallback

Signed-off-by: aarnautu <aarnautu@adobe.com>