fix(event-handler): avoid runtime import of metrics package in HTTP metrics middleware#5310
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svozza merged 6 commits intoJun 3, 2026
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…etrics middleware The HTTP metrics middleware did a value import of `MetricUnit` from `@aws-lambda-powertools/metrics`. Because `MetricUnit` is a runtime value, bundlers retained the import even for consumers that only use the `Router` and other middleware and never opt into metrics. Since the metrics package is an optional peer dependency, bundles without it installed crashed at init on Lambda with `ERR_MODULE_NOT_FOUND`. Inline the metric unit string literals instead, matching the type-only approach already used by the tracer middleware, so no runtime dependency on the metrics package leaks into Router-only bundles. Closes aws-powertools#5309
…uard Replace the comment-stripping and import-matching regexes in the metrics middleware import guard with a linear character scan and string checks, to clear the static-analysis security hotspots flagged on CI. The guard still fails if a runtime value import of the metrics package is reintroduced.
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Drop the hand-rolled comment-stripping scanner in favour of a plain statement split. Comments never trim to start with `import`, so they cannot cause a false positive, and splitting on the statement terminator keeps multi-line imports covered without any regex.
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Thank you for the quick PR - left a comment to remove the test case added in this PR.
Remove the import guard test per maintainer feedback.
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Summary
Changes
The HTTP metrics middleware (
packages/event-handler/src/http/middleware/metrics.ts) did a value import ofMetricUnitfrom@aws-lambda-powertools/metrics:Because
MetricUnitis a runtime value (not a type), bundlers such as esbuild retain this import even when the consuming code only uses theRouterand other middleware (e.g.cors) and never opts into the metrics middleware. The middleware barrel (http/middleware/index.ts) re-exportsmetricsalongsidecors, and the package does not declaresideEffects: false, so aRouter + cors-only bundle keepsmetrics.jsand itsMetricUnitimport.Since
@aws-lambda-powertools/metricsis an optional peer dependency, bundles that do not install it (and have no Powertools Lambda layer) crash at init on AWS Lambda withERR_MODULE_NOT_FOUND, returning HTTP 502 behind API Gateway.This change inlines the metric unit string literals (
'Milliseconds','Count') instead of importing theMetricUnitvalue. The literals are type-safe becauseaddMetric'sunitparameter is typed as the union of those exact string literals. This matches the type-only approach already used by the sibling tracer middleware (http/middleware/tracer.ts), so no runtime dependency on the metrics package leaks into Router-only bundles.A regression test asserts that the middleware source carries no value (non-
type) import from@aws-lambda-powertools/metrics. Verified that reintroducing the value import makes the test fail. Also verified manually by bundling aRouter + cors-only handler with esbuild (format: 'esm',bundle: true,platform: 'node'): after the change the output contains no reference to@aws-lambda-powertools/metrics.Issue number: closes #5309
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