docs(metrics): document InvokeStore isolation and local testing

[leiwilson]

Summary

Documents how Metrics behaves in the Lambda environment versus locally, including automatic per-invocation isolation via @aws/lambda-invoke-store when Lambda multi-concurrency is enabled, and clarifies how to assert emitted EMF metrics in unit tests.

Fixes #5187.

Changes

• docs/features/metrics.md: add "Lambda environment vs local" and retitle the existing spy example under "Asserting metrics in unit tests"

Test plan

• Skim Metrics docs Testing section for accuracy vs MetricsStore / shouldUseInvokeStore
• Confirm existing testing snippet still renders

Docs-only; no AWS credentials required.

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[boring-cyborg]

Thanks a lot for your first contribution! Please check out our contributing guidelines and don't hesitate to ask whatever you need.
In the meantime, check out the #typescript channel on our Powertools for AWS Lambda Discord: Invite link

[svozza]

Hi there, thanks for the contribution, can you add the acknowledgement section to the PR. You can see the wording in the GitHub template here: https://github.com/aws-powertools/powertools-lambda-typescript/blob/main/.github/PULL_REQUEST_TEMPLATE.md.

[leiwilson]

Thanks. The acknowledgment is in the description and the Acknowledgment Check is passing.

[svozza]

Can you address this comment too: #5486 (comment). Just needs a slight wording change to make the text clearer.

[leiwilson]

Thanks. Updated the wording so the in-memory store is described as the default on standard Lambda too, not only for local/tests.

[svozza]

One final tweak to the wording and then we're good to merge @leiwilson.

[dreamorosi]

@svozza I accepted your suggestion so you can approve.

[leiwilson]

Thanks — confirmed the wording now says Lambda on-demand functions.