feat(event-handler): add metrics middleware for HTTP routes

[svozza]

Summary

Add metrics middleware for emitting per-request metrics from the Event Handler HTTP router using the Powertools Metrics utility.

Changes

• Add route field to RequestContext populated with the matched route pattern (e.g. GET /users/:id), or null when no route matches
• Create metrics middleware that emits latency, fault, and error metrics per request
• Use the matched route as a metric dimension (uses NOT_FOUND when no route matches to prevent dimension explosion)
• Make RequestContext a discriminated union on responseType, enabling type-safe access to event-specific properties (e.g. requestContext.requestId on API Gateway events but not ALB)
• Add request metadata to metrics:
  • httpMethod and path for all requests
  • statusCode for all requests
  • ipAddress from requestContext.identity.sourceIp (V1), requestContext.http.sourceIp (V2), or X-Forwarded-For header (ALB fallback)
  • userAgent from request headers
  • apiGwRequestId and apiGwApiId for API Gateway V1 and V2 events
  • apiGwExtendedRequestId for API Gateway V1 events when available
• Remove unused getResponseType function (replaced by type guard branching in #buildRequestContext)
• Add @aws-lambda-powertools/metrics as an optional peer dependency, following the same pattern as the tracer middleware
• Expose the metrics middleware as a subpath export to avoid requiring the metrics dependency
• Unit tests are covered

Issue number: closes #4602

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[perpil]

x-forwarded-for has some gotchas

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
33.8% Duplication on New Code

See analysis details on SonarQube Cloud