fix(sdk): restore emit_metric as deprecated compat shim (#2850)
emit_metric was dropped without a backwards-compat shim when the Python
HTTP server was replaced by coglet. It's widely used across replicate/*
models (cog-whisper, cog-triton, cog-vllm, cog-arctic, cog-real-esrgan,
cog-proxy, etc.) and its absence causes a hard ImportError on startup.
Restore it as a shim that delegates to current_scope().record_metric(),
following the same print-to-stderr-once pattern established for
ExperimentalFeatureWarning. The deprecation message fires on first import
and is then cached in globals, so there's no overhead on subsequent calls.
