Support both histograms and summaries in distributions_as_summaries in Prometheus Sink

[jacobstr]

• We've got folks migrating from older aggregators / statsd to prometheus.
• They've come to rely on quantile generation.
• Quantiles have caveats when vector runs on multiple instances, they cannot be aggregated (avg(p95, p95) ... is not a "valid" p95).
• Histograms also render nicely in tools like grafana.
• I'd like to maintain continuity, while offering a forward-looking path to adopting histograms.

It'd be nice if this option supported either/or. The metric names/dimensions are different such that you could differentiate and have both on at once, I believe.