Histogram Metric Decorator

Framework-agnostic method decorator that records call duration into any histogram transport (Prometheus, OpenTelemetry, in-memory, …). Built on @rnw-community/decorators-core. Handles sync, Promise, and Observable return types with a single decorator. TypeScript experimentalDecorators.

When the observation fires

| Return | When | |---|---| | T (sync) | on return | | Promise<T> | on resolve | | Promise<T> that rejects | on reject (duration still emitted; error propagates) | | Observable<T> | on complete (completion-latency) | | Observable<T> that errors | on stream error (duration still emitted; error propagates) |

One decorator, correct duration semantics for all five shapes. Observable support is built in — rxjs is an optional peer, install it only when your methods return Observable.

Usage

yarn add @rnw-community/histogram-metric-decorator @rnw-community/decorators-core

import { createHistogramMetricDecorator } from '@rnw-community/histogram-metric-decorator';

import type { HistogramTransportInterface } from '@rnw-community/histogram-metric-decorator';

declare const transport: HistogramTransportInterface;

const HistogramMetric = createHistogramMetricDecorator({ transport });

class OrderService {
    @HistogramMetric()
    async placeOrder(order: Order): Promise<Receipt> { /* ... */ }

    @HistogramMetric({ name: 'order_fetch_ms', labels: ([id]) => ({ orderId: id }) })
    async fetchOrder(id: string): Promise<Order> { /* ... */ }
}

labels receives the method's args as a tuple — inferred from the method signature, no annotations needed. Both destructuring (labels: ([id]) => ({ orderId: id })) and indexed access (labels: (args) => ({ orderId: args[0] })) work; pick whichever reads clearer at the call site. Default metric name is `${className}_${methodName}_duration_ms`.

Wire any backend by implementing HistogramTransportInterface (observe(name, durationMs, labels?)); Prometheus and OpenTelemetry adapters are typically a few lines each. The package is transport-agnostic — consumers ship their own.

Crash-safe label resolution — onLabelsError

If the labels callback throws (for example because it dereferences a property on an optional argument that turned out to be undefined), the observation is still emitted — without labels — rather than crashing the decorated method. Pass onLabelsError to observe these throws for your diagnostics:

const HistogramMetric = createHistogramMetricDecorator({
    transport,
    onLabelsError: (err, args) => logger.warn({ err, args }, 'histogram label resolver threw'),
});

The onLabelsError hook itself is crash-safe — exceptions inside it are swallowed so a broken diagnostic never poisons the method it was observing.

Public API

• createHistogramMetricDecorator — factory; takes { transport, onLabelsError? } and returns <K extends AnyFn>(config?) => MethodDecoratorType<K>
• HistogramTransportInterface — observe(name, durationMs, labels?); implement for any backend
• HistogramOptionsInterface<TArgs> — per-decoration { name?, labels? }
• CreateHistogramMetricOptionsInterface — { transport, onLabelsError? }

License

MIT