Support a env var to configure default histogram buckets

[ktoso]

It may be common for service operators to try to tune the histogram buckets by trying it out in practice.

It may be nice to allow configuring this by SWIFT_PROMETHEUS_DEFAULT_HISTOGRAMS=0.1,0.2,0.5,.75,0.9 that would configure the bucket size unless they are configured explicitly in code.

[incertum]

+1 @ktoso would you like to open a draft PR for it? Happy to help reviewing it, especially looking closely at the security aspects (see also https://prometheus.io/docs/operating/security/).