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-".

dns/dns.go

@@ -10,10 +10,11 @@ import (
 	"golang.org/x/net/idna"
 
 	"github.com/mjl-/adns"
-
-	"github.com/mjl-/mox/moxvar"
 )
 
+// Pedantic enables stricter parsing.
+var Pedantic bool
+
 var (
 	errTrailingDot = errors.New("dns name has trailing dot")
 	errUnderscore  = errors.New("domain name with underscore")
@@ -113,7 +114,7 @@ func ParseDomain(s string) (Domain, error) {
 // is not enabled. Used for interoperability, e.g. domains may specify MX
 // targets with underscores.
 func ParseDomainLax(s string) (Domain, error) {
-	if moxvar.Pedantic || !strings.Contains(s, "_") {
+	if Pedantic || !strings.Contains(s, "_") {
 		return ParseDomain(s)
 	}
 

dns/resolver.go

@@ -12,12 +12,10 @@ import (
 
 	"golang.org/x/exp/slog"
 
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promauto"
-
 	"github.com/mjl-/adns"
 
 	"github.com/mjl-/mox/mlog"
+	"github.com/mjl-/mox/stub"
 )
 
 // todo future: replace with a dnssec capable resolver
@@ -29,18 +27,7 @@ func init() {
 }
 
 var (
-	metricLookup = promauto.NewHistogramVec(
-		prometheus.HistogramOpts{
-			Name:    "mox_dns_lookup_duration_seconds",
-			Help:    "DNS lookups.",
-			Buckets: []float64{0.001, 0.005, 0.01, 0.05, 0.100, 0.5, 1, 5, 10, 20, 30},
-		},
-		[]string{
-			"pkg",
-			"type",   // Lower-case Resolver method name without leading Lookup.
-			"result", // ok, nxdomain, temporary, timeout, canceled, error
-		},
-	)
+	MetricLookup stub.HistogramVec = stub.HistogramVecIgnore{}
 )
 
 // Resolver is the interface strict resolver implements.
@@ -106,7 +93,7 @@ func metricLookupObserve(pkg, typ string, err error, start time.Time) {
 	default:
 		result = "error"
 	}
-	metricLookup.WithLabelValues(pkg, typ, result).Observe(float64(time.Since(start)) / float64(time.Second))
+	MetricLookup.ObserveLabels(float64(time.Since(start))/float64(time.Second), pkg, typ, result)
 }
 
 func (r StrictResolver) WithPackage(name string) Resolver {