Allow the http_request_duration buckets to be configurable

[AdrianArnautu]

What is the underlying problem you're trying to solve?

http_request_duration has fixed, hardcoded number of buckets with no possibility to tweak them
see https://github.com/open-policy-agent/opa/blob/main/internal/prometheus/prometheus.go#L47

1e-6, // 1 microsecond
5e-6,
1e-5,
5e-5,
1e-4,
5e-4,
1e-3, // 1 millisecond
0.01,
0.1,
1, // 1 second

For cases when the most of the latencies are above 1ms, with only 4 available buckets we don't have a good insight on OPA's performance.
As a workaround, the client must to do the calculations.

Describe a "Good Enough" solution

Extend the OPA codebase to:

• accept a configurable number of buckets
• for this feature not to be a breaking change, we default to existing number of buckets if the configuration is missing

Additional Context

See this discussion: https://github.com/orgs/open-policy-agent/discussions/487

[AdrianArnautu]

I've already started the work on this, I'll submit a PR