chore: add tx metrics and logs for serialization errors (#15215)

Before db_metrics were all or nothing. Now `InTx` metrics are always recorded, and query metrics are opt in.


Adds instrumentation & logging around serialization failures in the database.

coderd/coderdtest/promhelp/doc.go

@@ -0,0 +1,3 @@
+// Package promhelp provides helper functions for asserting Prometheus
+// metric values in unit tests.
+package promhelp

coderd/coderdtest/promhelp/metrics.go

@@ -0,0 +1,87 @@
+package promhelp
+
+import (
+	"context"
+	"io"
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	ptestutil "github.com/prometheus/client_golang/prometheus/testutil"
+	io_prometheus_client "github.com/prometheus/client_model/go"
+	"github.com/stretchr/testify/require"
+)
+
+// RegistryDump returns the http page for a given registry's metrics.
+// Very useful for visual debugging.
+func RegistryDump(reg *prometheus.Registry) string {
+	h := promhttp.HandlerFor(reg, promhttp.HandlerOpts{})
+	rec := httptest.NewRecorder()
+	req, _ := http.NewRequestWithContext(context.Background(), http.MethodGet, "/", nil)
+	h.ServeHTTP(rec, req)
+	resp := rec.Result()
+	data, _ := io.ReadAll(resp.Body)
+	_ = resp.Body.Close()
+	return string(data)
+}
+
+// Compare can be used to compare a registry to some prometheus formatted
+// text. If any values differ, an error is returned.
+// If metric names are passed in, only those metrics will be compared.
+// Usage: `Compare(reg, RegistryDump(reg))`
+func Compare(reg prometheus.Gatherer, compare string, metricNames ...string) error {
+	return ptestutil.GatherAndCompare(reg, strings.NewReader(compare), metricNames...)
+}
+
+// HistogramValue returns the value of a histogram metric with the given name and labels.
+func HistogramValue(t testing.TB, reg prometheus.Gatherer, metricName string, labels prometheus.Labels) *io_prometheus_client.Histogram {
+	t.Helper()
+
+	labeled := MetricValue(t, reg, metricName, labels)
+	require.NotNilf(t, labeled, "metric %q with labels %v not found", metricName, labels)
+	return labeled.GetHistogram()
+}
+
+// GaugeValue returns the value of a gauge metric with the given name and labels.
+func GaugeValue(t testing.TB, reg prometheus.Gatherer, metricName string, labels prometheus.Labels) int {
+	t.Helper()
+
+	labeled := MetricValue(t, reg, metricName, labels)
+	require.NotNilf(t, labeled, "metric %q with labels %v not found", metricName, labels)
+	return int(labeled.GetGauge().GetValue())
+}
+
+// CounterValue returns the value of a counter metric with the given name and labels.
+func CounterValue(t testing.TB, reg prometheus.Gatherer, metricName string, labels prometheus.Labels) int {
+	t.Helper()
+
+	labeled := MetricValue(t, reg, metricName, labels)
+	require.NotNilf(t, labeled, "metric %q with labels %v not found", metricName, labels)
+	return int(labeled.GetCounter().GetValue())
+}
+
+func MetricValue(t testing.TB, reg prometheus.Gatherer, metricName string, labels prometheus.Labels) *io_prometheus_client.Metric {
+	t.Helper()
+
+	metrics, err := reg.Gather()
+	require.NoError(t, err)
+
+	for _, m := range metrics {
+		if m.GetName() == metricName {
+			for _, labeled := range m.GetMetric() {
+				mLabels := make(prometheus.Labels)
+				for _, v := range labeled.GetLabel() {
+					mLabels[v.GetName()] = v.GetValue()
+				}
+				if maps.Equal(mLabels, labels) {
+					return labeled
+				}
+			}
+		}
+	}
+	return nil
+}