feat: declaration-first usage metering (ms.Meter + Record)#122
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…by name) Declare each metric once in setup via ms.Meter(name, kind, ms.Unit/ms.Price) (side-effect registration into the manifest, no handle returned), then emit at runtime BY NAME with package-level ms.Record(ctx, name, value) — mirroring the existing ms.Emits/ms.Emit pair. No *Metric handle, no Gauge() method, no kind on the wire; the manifest-fed catalog is authoritative and the envelope stays v1. Guards: reserved infra.*/platform.* prefix + duplicate-name panic at declaration; undeclared-name + negative/non-finite value return an error from Record. Transport unchanged (MS_METER_LAMBDA_ARN / lambda.Invoke; dispatch-HTTP rewrite is PR #2). Fix defensive-nil bug: Record's dev-mode path (lambdaClient == nil) guards the logger before Printf, so a zero Client{} (or one from NewFromARN, which leaves logger nil) cannot panic. Adds a regression test. Design ref: docs-temp/milestone-d-meter/design.md §4/§5. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Declaration-first usage metering — BY NAME
A module declares each metric once in setup, then emits by name at runtime — exactly mirroring the existing
ms.Emits(declare) /ms.Emit(emit by name) pair. No stored handle, noGauge()method, nokindon the wire.API
ms.Meter(name string, kind Kind, opts ...MetricOption)— registers the metric into the module manifest as a side effect; returns nothing (no*Metrichandle).Kindisms.Counterorms.Gauge; options arems.Unit(...)andms.Price(microDollars)(price optional). Mirrors howms.Emitsregisters.ms.Record(ctx context.Context, name string, value float64) error— package-level, mirrorsms.Emit. Resolves the declared metric by name from the registry; returns an error if the name was never declared viams.Meter(declaration-first, fail fast). No chaining.Guards
infra.*/platform.*prefix → panic at declaration (platform-measured, not module-declarable).Record→ error.Record→ error (never panics the handler).Wire / transport
kindfield on the wire — the manifest-fed catalog is authoritative; no production emitters to stay compatible with).MS_METER_LAMBDA_ARN/lambda.Invoke; dev = stderr). The dispatch-HTTP rewrite is PR docs: add SDK package structure to README #2.Design ref
docs-temp/milestone-d-meter/design.md§4 (Axis 1) / §5 (app-module-sdk).Review follow-ups (non-blocking)
Recorddev-mode path now guards the nilloggerbeforePrintf, so a zeroClient{}(or one fromNewFromARN, which leavesloggernil) cannot panic. Regression test added.ValidateMetricNamedeliberately restatesregistry.ValidateNamebecausemeteris a public package that cannot importinternal/registry; a sharedinternal/namecheckcould dedup later if it becomes a maintenance burden.NewFromARNleavesloggernil by design (dev sink only); the invariant is now safe via the nil-guard rather than implicit.internal/core (m *Module).MetercallsDeclarethenAddMetricin lockstep from a single call site; manifest/registry stay in sync.TestRecord_DevModeLogsToStderris non-parallel (uses a shared logger sink); not a correctness issue.