add direct-to-prometheus metric for signer requests (#1095)

We could have misunderstood what the problem is by looking at a metric
that only changed before the first scrape.

Updates AUT-393
Updates AUT-199

handlers.go

@@ -17,9 +17,11 @@ import (
 	"net/url"
 	"os"
 	"path"
+	"strconv"
 	"time"
 
 	"github.com/gorilla/mux"
+	"github.com/prometheus/client_golang/prometheus"
 
 	"github.com/mozilla-services/autograph/formats"
 	"github.com/mozilla-services/autograph/signer"
@@ -217,6 +219,12 @@ func (a *autographer) handleSignature(w http.ResponseWriter, r *http.Request) {
 		}
 		requestedSignerConfig := requestedSigner.Config()
 		a.stats.Incr("signer.requests", []string{"keyid:" + requestedSignerConfig.ID, "user:" + userid, usedDefaultSignerTag(sigreq)}, 1.0)
+		signerRequestsCounter.With(prometheus.Labels{
+			"keyid": requestedSignerConfig.ID,
+			"user":  userid,
+			// TODO(AUT-206): remove this when we've migrate everyone off of the default keyid
+			"used_default_signer": strconv.FormatBool(sigreq.KeyID == ""),
+		}).Inc()
 
 		sigresps[i] = formats.SignatureResponse{
 			Ref:        id(),

stats.go

@@ -23,6 +23,11 @@ var (
 		Name: "prom_only_foobar_test",
 		Help: "A counter used for testing how prometheus and statsd metrics differ",
 	})
+
+	signerRequestsCounter = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Name: "signer_requests",
+		Help: "A counter for how many authenticated and authorized requests are made to a given signer",
+	}, []string{"keyid", "user", "used_default_signer"})
 )
 
 func loadStatsd(conf configuration) (*statsd.Client, error) {