Metrics

The latest value of every reading a hook takes, with where that value came from.

Recording

recordMetric(module, metric, value, source);

Each call stores a MetricRecord — module, metric, value, source and a timestamp — under the key module.metric, replacing whatever was there. 37 of the SDK’s source files record metrics directly.

Every traced call also records its own duration as <op>Ms, so how long useCPU.benchmarkCPU took is a metric without anyone writing that line.

Metrics are deliberately not logged per sample. A sensor at 10 Hz would otherwise put ten lines a second into the device log.

Reading

• getMetrics() — every current metric.
• metrics from useObservability() — the same, re-rendering as they change.
• getSourceSummary() — which provenance each module is currently reporting, built from the metrics. See Telemetry.

Limits

• Last value only. A new reading replaces the old one, so there is no history, no rate, no histogram and no percentile. A trend cannot be charted from the SDK.
• Untyped. value is unknown: a metric can hold a number, a string or an object, and nothing distinguishes a counter from a gauge.
• Memory only, like the rest of the layer.

These are listed with what closing them takes on the Observability overview.