stats: wire callback for background task

https://github.com/envoyproxy/envoy/pull/10097 added the ability to flush stats outside of the flushing interval. This initial feature allow us to do #573. However, flushing stats is not synchronous so right now the ability to flush is best-effort.

We don't plan on doing work in envoy to have a truly synchronous flushing path. However, what we can do to improve flushing probability during app backgrounding is set up a background task and have a version of flushStats in envoy that fires a callback when flushing is finished.