# Rules for working with AshMetrics ## Understanding AshMetrics AshMetrics is an Ash resource extension that adds a `metrics do` block in which counters, gauges and distributions are declared, and compiles those declarations into `Telemetry.Metrics` definitions for the reporter the host application already runs. Read `documentation/dsls/DSL-AshMetrics.md` before declaring metrics. It is generated from the DSL and lists every entity and every option. Apply the design test before adding anything to this package: would this work, unmodified, for a single-tenant Phoenix app exporting to Prometheus, with no Oban and no OpenTelemetry? If the answer is no, something app-specific has leaked in. ## Topics - `usage-rules/declaring_metrics.md` — the three primitives, tags and metric names. - `usage-rules/emitting.md` — which change or function emits a counter or a distribution. - `usage-rules/gauges.md` — filtering, grouping, cost and multitenancy. - `usage-rules/pollers_and_backends.md` — who polls a gauge, and where the value goes. - `usage-rules/configuration.md` — every configuration key, and when it is read. - `usage-rules/testing.md` — asserting on emissions.