Reduce the default collection interval from 2 to ~15 seconds

[czechboy0]

Most metrics backends don't collect/upload metrics more frequently than ~15-30 seconds, some (like swift-otel) even every 60 seconds - so it doesn't make sense for us to collect a bunch of gauges more frequently than that.

I think a better default here would be 15 seconds, but even less frequent might be enough (30 or 60).

That said, other backends, like swift-prometheus, are pull-based, so if a sidecar is collecting prom data from an instance every 5 seconds, the adopter will probably want to change the default collection interval to match that. So I still expect folks to customize it in both directions, I just think the default value doesn't need to be this low.