registry: add a multiplier option to convert values to base units (#2774)

Co-authored-by: Claude <noreply@anthropic.com>
Signed-off-by: Claude <noreply@anthropic.com>

docs/collector.registry.md

@@ -56,6 +56,7 @@ YAML:
       metric: windows_registry_windows_major_version # optional
       help: Windows major version number # optional
       type: gauge # optional
+      multiplier: 1 # optional
       labels: # optional
         product: windows
 - name: memory_management # required
@@ -77,6 +78,7 @@ JSON:
         "metric": "windows_registry_windows_major_version",
         "help": "Windows major version number",
         "type": "gauge",
+        "multiplier": 1,
         "labels": { "product": "windows" }
       }
     ]
@@ -157,6 +159,18 @@ in the registry.
 
 This key is optional.
 
+##### multiplier
+
+A factor that the value is multiplied by before it is exported, to convert it to
+the base unit the Prometheus [naming conventions](https://prometheus.io/docs/practices/naming/#base-units)
+ask for. For example, `0.001` converts milliseconds to seconds, `0.01` percent to
+a ratio and `3600` hours to seconds. It must be a positive number, so counters
+keep counting up. If the multiplier is the reciprocal of an integer, like `0.001`,
+the value is divided by that integer instead, which avoids rounding artifacts
+such as `0.009000000000000001`. If not specified, the value is exported as is.
+
+This key is optional.
+
 ##### labels
 
 Labels is a map of key-value pairs that will be added as constant labels to the

internal/collector/registry/registry.go

@@ -31,6 +31,7 @@ import (
 
 	"github.com/alecthomas/kingpin/v2"
 	"github.com/prometheus-community/windows_exporter/internal/mi"
+	"github.com/prometheus-community/windows_exporter/internal/multiplier"
 	"github.com/prometheus-community/windows_exporter/internal/types"
 	"github.com/prometheus/client_golang/prometheus"
 	"go.yaml.in/yaml/v3"
@@ -233,6 +234,15 @@ func (c *Collector) Build(logger *slog.Logger, _ *mi.Session) error {
 				continue
 			}
 
+			m, err := multiplier.New(value.Multiplier)
+			if err != nil {
+				errs = append(errs, fmt.Errorf("value %q of key %s: %w", value.Name, label, err))
+
+				continue
+			}
+
+			value.multiplier = m
+
 			help := value.Help
 			if help == "" {
 				help = "windows_exporter: custom registry metric"
@@ -369,7 +379,7 @@ func (c *Collector) collectKey(ch chan<- prometheus.Metric, key Key) error {
 		ch <- prometheus.MustNewConstMetric(
 			value.desc,
 			value.metricType,
-			float64(val),
+			value.multiplier.Apply(float64(val)),
 		)
 	}
 

internal/collector/registry/registry_output_test.go

@@ -78,6 +78,7 @@ func TestCollectorMetrics(t *testing.T) {
 	// 64-bit handling. Both are well under 2^53, so they convert to float64 exactly.
 	require.NoError(t, k.SetDWordValue("TestDword", 1234))
 	require.NoError(t, k.SetQWordValue("TestQword", 5_000_000_000))
+	require.NoError(t, k.SetDWordValue("TestMilliseconds", 9))
 	require.NoError(t, k.Close())
 
 	t.Cleanup(func() {
@@ -105,6 +106,13 @@ func TestCollectorMetrics(t *testing.T) {
 						Type:   "counter",
 						Labels: map[string]string{"foo": "bar"},
 					},
+					// The multiplier converts milliseconds to seconds; 9 * 0.001
+					// would be 0.009000000000000001.
+					{
+						Name:       "TestMilliseconds",
+						Metric:     "windows_registry_test_scaled_seconds",
+						Multiplier: new(0.001),
+					},
 				},
 			},
 			{
@@ -137,6 +145,9 @@ windows_registry_key_success{key="testgroup"} 1
 # HELP windows_registry_test_custom_metric custom registry help text
 # TYPE windows_registry_test_custom_metric counter
 windows_registry_test_custom_metric{foo="bar"} 5e+09
+# HELP windows_registry_test_scaled_seconds windows_exporter: custom registry metric
+# TYPE windows_registry_test_scaled_seconds gauge
+windows_registry_test_scaled_seconds 0.009
 # HELP windows_registry_testgroup_testdword windows_exporter: custom registry metric
 # TYPE windows_registry_testgroup_testdword gauge
 windows_registry_testgroup_testdword 1234

internal/collector/registry/registry_test.go

@@ -121,6 +121,16 @@ func TestCollectorBuildErrors(t *testing.T) {
 				},
 			}},
 		},
+		{
+			name: "invalid multiplier",
+			config: registry.Config{Keys: []registry.Key{
+				{
+					Name:   "windows_nt",
+					Key:    `HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion`,
+					Values: []registry.Value{{Name: "CurrentMajorVersionNumber", Multiplier: new(0.0)}},
+				},
+			}},
+		},
 		{
 			name: "shared metric name with inconsistent help text",
 			config: registry.Config{Keys: []registry.Key{

internal/collector/registry/types.go

@@ -19,6 +19,7 @@ package registry
 
 import (
 	"github.com/prometheus-community/windows_exporter/internal/config/yamlstring"
+	"github.com/prometheus-community/windows_exporter/internal/multiplier"
 	"github.com/prometheus/client_golang/prometheus"
 	"go.yaml.in/yaml/v3"
 	winregistry "golang.org/x/sys/windows/registry"
@@ -36,15 +37,17 @@ type Key struct {
 }
 
 type Value struct {
-	Name   string            `json:"name"   yaml:"name"`
-	Metric string            `json:"metric" yaml:"metric"`
-	Help   string            `json:"help"   yaml:"help"`
-	Type   string            `json:"type"   yaml:"type"`
-	Labels map[string]string `json:"labels" yaml:"labels"`
+	Name       string            `json:"name"       yaml:"name"`
+	Metric     string            `json:"metric"     yaml:"metric"`
+	Help       string            `json:"help"       yaml:"help"`
+	Type       string            `json:"type"       yaml:"type"`
+	Labels     map[string]string `json:"labels"     yaml:"labels"`
+	Multiplier *float64          `json:"multiplier" yaml:"multiplier"`
 
 	// resolved at Build time
 	desc       *prometheus.Desc
 	metricType prometheus.ValueType
+	multiplier multiplier.Multiplier
 }
 
 // UnmarshalYAML decodes the registry block of the configuration file, where

internal/multiplier/multiplier.go

@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: Apache-2.0
+//
+// Copyright The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package multiplier applies the optional multiplier of a user-defined metric
+// of the wmi, performancecounter and registry collectors. It converts values
+// to base units, for example milliseconds to seconds with 0.001.
+package multiplier
+
+import (
+	"fmt"
+	"math"
+)
+
+// maxExactInteger is the largest integer up to which every integer has an
+// exact float64 representation.
+const maxExactInteger = 1 << 53
+
+// Multiplier scales values. The zero value leaves them unchanged.
+type Multiplier struct {
+	factor  float64
+	divisor float64
+}
+
+// New validates the multiplier of the configuration; nil means none. It must
+// be a positive finite number, so that counters stay monotonic.
+func New(multiplier *float64) (Multiplier, error) {
+	if multiplier == nil {
+		return Multiplier{}, nil
+	}
+
+	m := *multiplier
+
+	if math.IsNaN(m) || math.IsInf(m, 0) || m <= 0 {
+		return Multiplier{}, fmt.Errorf("multiplier must be a positive finite number, got %v", m)
+	}
+
+	if m == 1 {
+		return Multiplier{}, nil
+	}
+
+	// x * 0.001 is not always the correctly rounded x / 1000, because 0.001
+	// has no exact binary representation: 9 * 0.001 is 0.009000000000000001.
+	// Divide by the reciprocal instead when it is an integer.
+	if m < 1 {
+		if d := math.Round(1 / m); d <= maxExactInteger && 1/d == m {
+			return Multiplier{divisor: d}, nil
+		}
+	}
+
+	return Multiplier{factor: m}, nil
+}
+
+// Apply returns v scaled by the multiplier.
+func (m Multiplier) Apply(v float64) float64 {
+	switch {
+	case m.divisor != 0:
+		return v / m.divisor
+	case m.factor != 0:
+		return v * m.factor
+	default:
+		return v
+	}
+}

internal/multiplier/multiplier_test.go

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: Apache-2.0
+//
+// Copyright The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package multiplier_test
+
+import (
+	"math"
+	"testing"
+
+	"github.com/prometheus-community/windows_exporter/internal/multiplier"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNew(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		name       string
+		multiplier *float64
+		value      float64
+		want       float64
+	}{
+		{"unset", nil, 42, 42},
+		{"one", new(1.0), 42, 42},
+		{"hours to seconds", new(3600.0), 2, 7200},
+		{"megabytes to bytes", new(1048576.0), 3, 3145728},
+		{"milliseconds to seconds", new(0.001), 9, 0.009},
+		{"percent to ratio", new(0.01), 7, 0.07},
+		{"ticks to seconds", new(1e-7), 115960642, 11.5960642},
+		{"non-integer reciprocal", new(0.4), 5, 2},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			m, err := multiplier.New(tc.multiplier)
+			require.NoError(t, err)
+			require.Equal(t, tc.want, m.Apply(tc.value)) //nolint:testifylint // The result must be exact.
+		})
+	}
+}
+
+func TestNewInvalid(t *testing.T) {
+	t.Parallel()
+
+	for _, m := range []float64{0, -1, math.NaN(), math.Inf(1), math.Inf(-1)} {
+		_, err := multiplier.New(&m)
+		require.Error(t, err, "multiplier %v", m)
+	}
+}
+
+// TestApplyDividesExactly checks that a multiplier that is the reciprocal of
+// an integer gives the correctly rounded quotient, which a plain
+// multiplication misses for about one in seven values.
+func TestApplyDividesExactly(t *testing.T) {
+	t.Parallel()
+
+	for _, divisor := range []float64{10, 100, 1000, 1024, 1e6, 1e7, 1e9} {
+		m, err := multiplier.New(new(1 / divisor))
+		require.NoError(t, err)
+
+		for i := range 100000 {
+			v := float64(i)
+			if got := m.Apply(v); got != v/divisor {
+				t.Fatalf("Apply(%v) with multiplier 1/%v = %v, want %v", v, divisor, got, v/divisor)
+			}
+		}
+	}
+}