expose fewer internals in packages, for easier software reuse

- prometheus is now behind an interface, they aren't dependencies for the
  reusable components anymore.
- some dependencies have been inverted: instead of packages importing a main
  package to get configuration, the main package now sets configuration in
  these packages. that means fewer internals are pulled in.
- some functions now have new parameters for values that were retrieved from
  package "mox-".

iprev/iprev.go

@@ -11,24 +11,15 @@ import (
 
 	"golang.org/x/exp/slog"
 
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promauto"
-
 	"github.com/mjl-/mox/dns"
 	"github.com/mjl-/mox/mlog"
+	"github.com/mjl-/mox/stub"
 )
 
 var xlog = mlog.New("iprev", nil)
 
 var (
-	metricIPRev = promauto.NewHistogramVec(
-		prometheus.HistogramOpts{
-			Name:    "mox_iprev_lookup_total",
-			Help:    "Number of iprev lookups.",
-			Buckets: []float64{0.001, 0.005, 0.01, 0.05, 0.100, 0.5, 1, 5, 10, 20, 30},
-		},
-		[]string{"status"},
-	)
+	MetricIPRev stub.HistogramVec = stub.HistogramVecIgnore{}
 )
 
 // Lookup errors.
@@ -62,7 +53,7 @@ func Lookup(ctx context.Context, resolver dns.Resolver, ip net.IP) (rstatus Stat
 	log := xlog.WithContext(ctx)
 	start := time.Now()
 	defer func() {
-		metricIPRev.WithLabelValues(string(rstatus)).Observe(float64(time.Since(start)) / float64(time.Second))
+		MetricIPRev.ObserveLabels(float64(time.Since(start))/float64(time.Second), string(rstatus))
 		log.Debugx("iprev lookup result", rerr,
 			slog.Any("ip", ip),
 			slog.Any("status", rstatus),