feat(flags-core): add metrics environment option (#453)

* feat(flags-core): support explicit environment

Co-Authored-By: Claude <noreply@anthropic.com>

Co-Authored-By: Luis Meyer <luis.meyer@vercel.com>

* fix(flags-core): scope environment to metrics ingest

Co-Authored-By: Claude <noreply@anthropic.com>

Co-Authored-By: Luis Meyer <luis.meyer@vercel.com>

* fix(flags-core): clarify metrics environment

Co-Authored-By: Claude <noreply@anthropic.com>

* fix(flags-core): restrict metric environment values

* docs(flags-core): clarify metrics environment fallback

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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>