[chore] Validate metadata.yaml units against UCUM specification (#15530)

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#### Description

Validate metrics units using `github.com/iimos/ucum`.

Depends on open-telemetry/opentelemetry-collector-contrib/pull/49453

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Author: mx-psi

cmd/mdatagen/go.mod

@@ -4,6 +4,7 @@ go 1.25.0
 
 require (
 	github.com/google/go-cmp v0.7.0
+	github.com/iimos/ucum v0.0.3
 	github.com/spf13/cobra v1.10.2
 	github.com/stretchr/testify v1.11.1
 	go.opentelemetry.io/collector/component v1.62.0

cmd/mdatagen/go.sum

@@ -9,6 +9,7 @@ import (
 	"regexp"
 	"strings"
 
+	"github.com/iimos/ucum"
 	"golang.org/x/text/cases"
 	"golang.org/x/text/language"
 
@@ -96,6 +97,10 @@ func (m *Metric) validate(metricName MetricName, semConvVersion string) error {
 	}
 	if m.Unit == nil {
 		errs = errors.Join(errs, errors.New(`missing metric unit`))
+	} else if *m.Unit != "" {
+		if _, err := ucum.Parse([]byte(*m.Unit)); err != nil {
+			errs = errors.Join(errs, fmt.Errorf("invalid metric unit %q: %w", *m.Unit, err))
+		}
 	}
 	if m.Sum != nil {
 		errs = errors.Join(errs, m.Sum.Validate())

cmd/mdatagen/internal/metric.go

@@ -9,9 +9,54 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/pdata/pmetric"
 )
 
+func TestMetricValidateUnit(t *testing.T) {
+	strPtr := func(s string) *string { return &s }
+	newMetric := func(unit *string) *Metric {
+		return &Metric{
+			Signal: Signal{
+				Description: "a metric",
+				Stability:   component.StabilityLevelBeta,
+			},
+			Unit: unit,
+			Sum: &Sum{
+				MetricValueType: MetricValueType{pmetric.NumberDataPointValueTypeInt},
+				Mono:            Mono{Monotonic: true},
+			},
+		}
+	}
+	tests := []struct {
+		name    string
+		unit    *string
+		wantErr string
+	}{
+		{name: "valid seconds", unit: strPtr("s")},
+		{name: "valid dimensionless", unit: strPtr("1")},
+		{name: "valid bytes", unit: strPtr("By")},
+		{name: "valid percent", unit: strPtr("%")},
+		{name: "valid annotation", unit: strPtr("{packets}")},
+		{name: "valid compound", unit: strPtr("By/s")},
+		{name: "empty unit allowed", unit: strPtr("")},
+		{name: "missing unit", unit: nil, wantErr: "missing metric unit"},
+		{name: "non-ucum micro sign", unit: strPtr("µs"), wantErr: `invalid metric unit "µs"`},
+		{name: "unknown unit", unit: strPtr("bytes"), wantErr: `invalid metric unit "bytes"`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := newMetric(tt.unit).validate("metric.name", "")
+			if tt.wantErr != "" {
+				require.Error(t, err)
+				assert.ErrorContains(t, err, tt.wantErr)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}
+
 func TestValidateSemConvMetricURL(t *testing.T) {
 	validURL := "https://github.com/open-telemetry/semantic-conventions/blob/v1.37.2/docs/system/system-metrics.md#metric-systemcputime"
 	tests := []struct {

cmd/mdatagen/internal/metric_test.go

@@ -8,6 +8,6 @@ telemetry:
   metrics:
     sampling_decision_latency:
       description: Latency (in microseconds) of a given sampling policy
-      unit: µs
+      unit: us
       enabled: true
       histogram:

cmd/mdatagen/internal/testdata/invalid_telemetry_missing_value_type_for_histogram.yaml

@@ -12,7 +12,7 @@ telemetry:
   metrics:
     sampling_decision_latency:
       description: Latency (in microseconds) of a given sampling policy
-      unit: µs
+      unit: us
       enabled: true
       stability: alpha
       histogram: