The main motivation here is to not have `splitter` import from `bgs`; it was only doing so for MetricsMiddleware. This moves MetricsMiddleware out to new service-oriented util package (not directly in `util/` because it depends on prometheus and echo), and updates the trivial usage to that. Not touching `cmd/relay/` because that is getting refactored in a separate branch; and not touching `search/metrics.go` because it adds "extra" labels.