feat(telemetry): BusinessMetrics — transparently tenant-scoped instruments

[pitabwire]

Standard factory for business metrics where tenant_id/partition_id attach automatically from the security claims context on every Add/Record. Foundation for the fleet-wide analytics standardization. Test proves: tenant attrs present with claims, absent without, explicit attrs preserved.

[gemini-code-assist]

Code Review

This pull request introduces a new BusinessMetrics factory in telemetry/business.go along with corresponding tests in telemetry/business_test.go. The factory transparently attaches tenant and partition attributes from the context to various metric types (Counter, FloatCounter, Histogram, Gauge, FloatGauge). The review feedback suggests optimizing the merged function to avoid unnecessary slice allocations when either tenant or explicit attributes are empty. Additionally, it recommends expanding the test suite to fully cover and verify the newly introduced Gauge and FloatGauge metrics.

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[gemini-code-assist]

On high-throughput hot paths, minimizing slice allocations is crucial for performance. The current implementation of merged always allocates a new slice all when either tenant or attrs is non-empty, even if one of them is completely empty.

We can optimize this by returning metric.WithAttributes directly with the non-empty slice when the other is empty, avoiding any new slice allocation in those very common scenarios.

func merged(ctx context.Context, attrs []attribute.KeyValue) metric.MeasurementOption {
	tenant := TenantAttributes(ctx)
	if len(tenant) == 0 {
		if len(attrs) == 0 {
			return metric.WithAttributes()
		}
		return metric.WithAttributes(attrs...)
	}
	if len(attrs) == 0 {
		return metric.WithAttributes(tenant...)
	}
	all := make([]attribute.KeyValue, 0, len(tenant)+len(attrs))
	all = append(all, tenant...)
	all = append(all, attrs...)
	return metric.WithAttributes(all...)
}

[gemini-code-assist]

The helper function dataPointAttrs currently only handles Sum and Histogram aggregations. It does not support Gauge aggregations, which means any future tests for Gauge or FloatGauge would return nil attributes and fail to verify tenant-scoping.

Adding support for metricdata.Gauge[int64] and metricdata.Gauge[float64] makes this helper complete and robust.

func dataPointAttrs(data metricdata.Aggregation) []attribute.Set {
	switch d := data.(type) {
	case metricdata.Sum[int64]:
		out := make([]attribute.Set, 0, len(d.DataPoints))
		for _, dp := range d.DataPoints {
			out = append(out, dp.Attributes)
		}
		return out
	case metricdata.Sum[float64]:
		out := make([]attribute.Set, 0, len(d.DataPoints))
		for _, dp := range d.DataPoints {
			out = append(out, dp.Attributes)
		}
		return out
	case metricdata.Histogram[float64]:
		out := make([]attribute.Set, 0, len(d.DataPoints))
		for _, dp := range d.DataPoints {
			out = append(out, dp.Attributes)
		}
		return out
	case metricdata.Gauge[int64]:
		out := make([]attribute.Set, 0, len(d.DataPoints))
		for _, dp := range d.DataPoints {
			out = append(out, dp.Attributes)
		}
		return out
	case metricdata.Gauge[float64]:
		out := make([]attribute.Set, 0, len(d.DataPoints))
		for _, dp := range d.DataPoints {
			out = append(out, dp.Attributes)
		}
		return out
	default:
		return nil
	}
}

[gemini-code-assist]

The test currently only verifies Counter, FloatCounter, and Histogram. It should also verify that Gauge and FloatGauge are correctly created and transparently carry the tenant attributes.

Suggested change

	bm := telemetry.NewBusinessMetrics("biz-test")
	created := bm.Counter("things_created_total", "things created")
	amount := bm.FloatCounter("things_amount_total", "amount of things")
	dur := bm.Histogram("things_duration_ms", "thing duration")
	ctxA := tenantContext("tenant-a", "part-a")
	created.Add(ctxA, 1, attribute.String("kind", "gold"))
	amount.Add(ctxA, 12.5)
	dur.Record(ctxA, 42.0)
	bm := telemetry.NewBusinessMetrics("biz-test")
	created := bm.Counter("things_created_total", "things created")
	amount := bm.FloatCounter("things_amount_total", "amount of things")
	dur := bm.Histogram("things_duration_ms", "thing duration")
	gauge := bm.Gauge("things_gauge", "gauge of things")
	fgauge := bm.FloatGauge("things_fgauge", "float gauge of things")
	ctxA := tenantContext("tenant-a", "part-a")
	created.Add(ctxA, 1, attribute.String("kind", "gold"))
	amount.Add(ctxA, 12.5)
	dur.Record(ctxA, 42.0)
	gauge.Record(ctxA, 100)
	fgauge.Record(ctxA, 100.5)

[gemini-code-assist]

Include the newly added things_gauge and things_fgauge in the verification loop to ensure they correctly carry the tenant attributes.

Suggested change

	for _, name := range []string{"things_amount_total", "things_duration_ms"} {
	s := attrSets(rm, name)
	require.NotEmpty(t, s, name)
	v, ok := s[0].Value("tenant_id")
	require.True(t, ok, name+" must carry tenant_id")
	require.Equal(t, "tenant-a", v.AsString())
	}
	for _, name := range []string{"things_amount_total", "things_duration_ms", "things_gauge", "things_fgauge"} {
	s := attrSets(rm, name)
	require.NotEmpty(t, s, name)
	v, ok := s[0].Value("tenant_id")
	require.True(t, ok, name+" must carry tenant_id")
	require.Equal(t, "tenant-a", v.AsString())
	}